The client should not consume so much when no synchronization is performed.
After upgrading to client v2.5, Nextcloud _always_ consumes 10-25% of CPU. Even minimized to tray with paused synchronization.
Client version: Version 2.5.0v2.5.0 (build 20181112) Built from Git revision 371001 on Nov 12 2018, 11:38:51 using Qt 5.11.1, OpenSSL 1.0.1h 5 Jun 2014
Operating system: Windows 7 (64-Bit)
OS language: English
Installation path of client: C:\Program Files (x86)\Nextcloud
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "dcfe8db1-ab98-45b6-a081-3d279ade04b8"
[OCC::AbstractNetworkJob::start OCC::PropfindJob created for "https://MY_NEXTCLOUD_SERVER_1" + "/" "OCC::QuotaInfo"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::PropfindJob::finished PROPFIND of QUrl("https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::Folder::slotRunEtagJob Trying to check "https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/" for changes via ETag check. (time since last sync: 457 s)
[OCC::Folder::slotRunEtagJob Trying to check "https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/" for changes via ETag check. (time since last sync: 456 s)
[OCC::Folder::slotRunEtagJob Trying to check "https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/" for changes via ETag check. (time since last sync: 37 s)
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "1c3fb88c-d91d-48f8-b63b-6c006025a0cf"
[OCC::AbstractNetworkJob::start OCC::RequestEtagJob created for "https://MY_NEXTCLOUD_SERVER_1" + "/" "OCC::Folder"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::RequestEtagJob::finished Request Etag of QUrl("https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "3f66dc85-f48a-4d2b-b34c-f63b29d9f984"
[OCC::AbstractNetworkJob::start OCC::RequestEtagJob created for "https://MY_NEXTCLOUD_SERVER_2" + "/" "OCC::Folder"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::RequestEtagJob::finished Request Etag of QUrl("https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "5a299fbf-c2bf-4d46-ad51-ca457f99d295"
[OCC::AbstractNetworkJob::start OCC::RequestEtagJob created for "https://MY_NEXTCLOUD_SERVER_3" + "/" "OCC::Folder"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::RequestEtagJob::finished Request Etag of QUrl("https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "d2b93c47-4dc0-47a4-85b6-559f00bbf7bb"
[OCC::AbstractNetworkJob::start OCC::PropfindJob created for "https://MY_NEXTCLOUD_SERVER_1" + "/" "OCC::ConnectionValidator"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "6213be97-23b1-464c-b3ff-2e77e3f31b33"
[OCC::AbstractNetworkJob::start OCC::PropfindJob created for "https://MY_NEXTCLOUD_SERVER_2" + "/" "OCC::ConnectionValidator"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "273472ac-f6c9-4618-8922-e2ef4c4a4b5e"
[OCC::AbstractNetworkJob::start OCC::PropfindJob created for "https://MY_NEXTCLOUD_SERVER_3" + "/" "OCC::ConnectionValidator"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::PropfindJob::finished PROPFIND of QUrl("https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::PropfindJob::finished PROPFIND of QUrl("https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::PropfindJob::finished PROPFIND of QUrl("https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "a3dc1727-537d-4d75-a373-21ce36e9c48f"
[OCC::AbstractNetworkJob::start OCC::PropfindJob created for "https://MY_NEXTCLOUD_SERVER_1" + "/" "OCC::QuotaInfo"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::PropfindJob::finished PROPFIND of QUrl("https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::Folder::slotRunEtagJob Trying to check "https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/" for changes via ETag check. (time since last sync: 487 s)
[OCC::Folder::slotRunEtagJob Trying to check "https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/" for changes via ETag check. (time since last sync: 486 s)
[OCC::Folder::slotRunEtagJob Trying to check "https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/" for changes via ETag check. (time since last sync: 67 s)
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "e58a0373-cd79-4ff6-989c-c7eb7426d294"
[OCC::AbstractNetworkJob::start OCC::RequestEtagJob created for "https://MY_NEXTCLOUD_SERVER_1" + "/" "OCC::Folder"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::RequestEtagJob::finished Request Etag of QUrl("https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "360e9dc4-1763-43b9-87d0-780e81a73fee"
[OCC::AbstractNetworkJob::start OCC::RequestEtagJob created for "https://MY_NEXTCLOUD_SERVER_2" + "/" "OCC::Folder"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::RequestEtagJob::finished Request Etag of QUrl("https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "46561c99-8ec8-442a-ade3-a20b62035fcc"
[OCC::AbstractNetworkJob::start OCC::RequestEtagJob created for "https://MY_NEXTCLOUD_SERVER_3" + "/" "OCC::Folder"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::RequestEtagJob::finished Request Etag of QUrl("https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "34c77a22-be25-412d-8ddc-1391059edaec"
[OCC::AbstractNetworkJob::start OCC::PropfindJob created for "https://MY_NEXTCLOUD_SERVER_1" + "/" "OCC::ConnectionValidator"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "7835be7b-405e-4fb1-a832-621f6b7d42fa"
[OCC::AbstractNetworkJob::start OCC::PropfindJob created for "https://MY_NEXTCLOUD_SERVER_2" + "/" "OCC::ConnectionValidator"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "dc3263e8-289d-41ec-b24a-ece7701276ee"
[OCC::AbstractNetworkJob::start OCC::PropfindJob created for "https://MY_NEXTCLOUD_SERVER_3" + "/" "OCC::ConnectionValidator"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::PropfindJob::finished PROPFIND of QUrl("https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::PropfindJob::finished PROPFIND of QUrl("https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::PropfindJob::finished PROPFIND of QUrl("https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "320650e0-b0d0-4ca5-907b-58d20297353a"
[OCC::AbstractNetworkJob::start OCC::PropfindJob created for "https://MY_NEXTCLOUD_SERVER_1" + "/" "OCC::QuotaInfo"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::PropfindJob::finished PROPFIND of QUrl("https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::Folder::slotRunEtagJob Trying to check "https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/" for changes via ETag check. (time since last sync: 517 s)
[OCC::Folder::slotRunEtagJob Trying to check "https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/" for changes via ETag check. (time since last sync: 516 s)
[OCC::Folder::slotRunEtagJob Trying to check "https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/" for changes via ETag check. (time since last sync: 97 s)
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "03b659dc-0931-4847-b8e5-6918316f1192"
[OCC::AbstractNetworkJob::start OCC::RequestEtagJob created for "https://MY_NEXTCLOUD_SERVER_1" + "/" "OCC::Folder"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::RequestEtagJob::finished Request Etag of QUrl("https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "98a475e2-6f78-49b1-a333-4400e4a7c942"
[OCC::AbstractNetworkJob::start OCC::RequestEtagJob created for "https://MY_NEXTCLOUD_SERVER_2" + "/" "OCC::Folder"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::RequestEtagJob::finished Request Etag of QUrl("https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "8729b213-7463-4350-bccc-64d6bc22f9b7"
[OCC::AbstractNetworkJob::start OCC::RequestEtagJob created for "https://MY_NEXTCLOUD_SERVER_3" + "/" "OCC::Folder"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::RequestEtagJob::finished Request Etag of QUrl("https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "ed2bbcaa-c3ae-45d7-90e8-4fe9f2887c10"
[OCC::AbstractNetworkJob::start OCC::PropfindJob created for "https://MY_NEXTCLOUD_SERVER_1" + "/" "OCC::ConnectionValidator"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "c9eeac5f-c67c-4dfd-a28a-13c475e8c064"
[OCC::AbstractNetworkJob::start OCC::PropfindJob created for "https://MY_NEXTCLOUD_SERVER_2" + "/" "OCC::ConnectionValidator"
[OCC::AccessManager::createRequest 6 "PROPFIND" "https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/" has X-Request-ID "28365020-11ff-4b93-9541-12f5590df3ae"
[OCC::AbstractNetworkJob::start OCC::PropfindJob created for "https://MY_NEXTCLOUD_SERVER_3" + "/" "OCC::ConnectionValidator"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::PropfindJob::finished PROPFIND of QUrl("https://MY_NEXTCLOUD_SERVER_2/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::PropfindJob::finished PROPFIND of QUrl("https://MY_NEXTCLOUD_SERVER_3/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
[OCC::WebFlowCredentials::slotFinished request finished
[OCC::WebFlowCredentials::stillValid Still valid?
[OCC::WebFlowCredentials::stillValid QNetworkReply::NetworkError(NoError)
[OCC::WebFlowCredentials::stillValid "Unknown error"
[OCC::PropfindJob::finished PROPFIND of QUrl("https://MY_NEXTCLOUD_SERVER_1/remote.php/dav/files/MY_USERNAME/") FINISHED WITH STATUS "OK"
Similar issue on Linux, with the difference it consumes about 100% on a single thread for quite a long time.
Same behavior on my Win7/64 with the current build 2018112 - didn't observe that with the old 2.3 client.
I have a similar symptom since update to the latest version 2.5.0 (build 20181112).
When the Nextcloud client is running, the svchost.exe process consumes constantly 20-24 % CPU and the nextcloud.exe consumes about 2-4 % CPU.
I don't see anything special in the logfile. My machine has a quad-core processor.
Windows 7 Enterprise, SP1, 64 Bit, English with german language pack.

I'm having this issue on Linux as well, using the latest build from git available via the ppa.
Same issue with Version 2.5.1final (build 20181204).
The process consumes around 14% of CPU on an i7-4600U
I just confirmed that the issue is related to the wallet service being used on Linux (I think Kwallet in this case). I'll open a separate issue.
Can confirm, same issue on OpenSUSE TW with KDE (kwallet)
Built from Git revision b37cbe using Qt 5.12.0, OpenSSL 1.1.0h-fips 27 Mar 2018
Having the same issue with 2.5.1 (from ArchLinux AUR, 4 core cpu), additionally it's consuming 2,2GB RAM.
Nothing exciting in the logs, but high CPU usage happens when it starts to sync files and lasts a couple of minutes. I have thousands of ignored files, maybe that is causing an issue? My Nextcloud sync has about 50k files total.
Wasn't a problem before the latest update, but I can't say which version I was running before.

Same here. We just switched over from ownCloud, but that issue makes me want to go back there.
I can confirm that the problem seems to be in QtWebengine as described in #964.
Before:

After opening any webview (e.g. "Account" -> "Add new" -> "Register with a provider") in the app the CPU usage drops to normal values (But the use of memory is increasing. And that memory isn't free after closing the webview.):

I second this bug:
see the attachment with CPU load before (0->3/4 of time) and after I shut down the client
Windows 7 Enterprise 64bit, i7-6600U

Also here
v. 2.6.0 stable win 64 b20190927
Same here. 7-13% CPU usage at all times, always trying to sync something even though there are no changes - see attached gif.
similar problem on osX 10.15 with nextcloud client 2.6.0
https://github.com/nextcloud/desktop/issues/1550
same on 2.6.1 on osx 10.15.2.
Most helpful comment
I can confirm that the problem seems to be in QtWebengine as described in #964.
Before:

After opening any webview (e.g. "Account" -> "Add new" -> "Register with a provider") in the app the CPU usage drops to normal values (But the use of memory is increasing. And that memory isn't free after closing the webview.):
