The transitions are very sluggish while hovering over PRs. Would love to see them become more smooth and help if required.

Also, sorry if this is not the right place to ask this, but is there a way to disable this collapse behaviour? I couldn't find the name of this feature in the list of features.
Confirmed. This was broken by the last “fix” we applied to that feature. PR welcome to revert that and/or fix it again, but preserving the way it was working before.
Most helpful comment
The feature is called
dim-bots:https://github.com/sindresorhus/refined-github/blob/master/source/features/dim-bots.tsx
https://github.com/sindresorhus/refined-github/blob/master/source/features/dim-bots.css