Mbed-os: Poor stdio performance on μARM

[neilt6]

File I/O performance is currently very poor when using FATFileSystem on the μARM toolchain. This is made evident by the attached SD card benchmarks. More details are available here.

[ciarmcom]

ARM Internal Ref: IOTMORF-322

[kegilbert]

@geky

[geky]

We're looking into the performance with the updated SD card implementation over here: https://github.com/ARMmbed/mbed-os/pull/4207

Additionally, there's been quite a bit of work around the Filesystem APIs, as well as removal of uARM as a toolchain, so I wonder if these performance issues are still present.

[deepikabhavnani]

Closing this, please reopen if issue still exists with latest driver.