SUMMARY
(This is actually not a kcm_lookandfeel bug, rather kcm_lookandfeel is the only module that works properly)
On X11 with a 165Hz monitor and Nvidia, the sidebar animation when a sidebar item is clicked stutters as if rendered on a much lower framerate. Interestingly, the animation into Global Themes is exceptionally smooth and after clicking into Global Themes and exiting, all such animations become butter smooth -- as if some magic flag has been set globally.

STEPS TO REPRODUCE
1. Launch systemsettings.
2. Click into categories other than 'Appearance,' observe how the animation stutters.
3. Click into Appearance (then Global Theme, which is automatically selected).
4. Now click into whatever categories you like, the animation becomes butter smooth.

EXPECTED RESULT
The animations should be smooth in the first place.

SOFTWARE/OS VERSIONS
Linux: Arch Linux
KDE Plasma Version: 5.25.90
KDE Frameworks Version: 5.98.0
Qt Version: 5.15.6

ADDITIONAL INFORMATION
I initially thought it was a problem with the render loop and tried launch systemsettings by
QSG_RENDER_LOOP=threaded systemsettings
but turns out it didn't work.

Almost all animations in Plasma runs smoothly at 165Hz on my machine . The one reported here and the GNOME-like overview animation are two exceptions. Both appears to be at 60Hz initially.