When using Plasma 6.1, the general performance of Plasma is having various framerate issues. 
My laptop is equipped with a 165hz screen and when using testufo.com the framerate appears to fluctuate between 100, 90, or even something like 60 fps. Sometimes it stabilizes for a short time at the correct 165 fps, only to fall back to the inconsistent FPS again. 

This happens on the internal laptop screen without any external screen connected.

STEPS TO REPRODUCE
1. Open testufo.com in Kwin 6.0.90.1 using Firefox (using it's wayland backend)
2. 
3. 

OBSERVED RESULT

Framerates should vary below the monitor refresh rate.

EXPECTED RESULT

The Framerate should be the same as the monitor refresh rate

SOFTWARE/OS VERSIONS
Linux/KDE Plasma:  6.9.2-arch1-1
(available in About System)
KDE Plasma Version: 6.0.90
KDE Frameworks Version: 6.2.0-1
Qt Version: 6.7.1

ADDITIONAL INFORMATION

I've tested against Mesa 24.0.7 and 24.1.0, and both show the same behavior. 
Using kwin-explicit-sync from AUR (https://aur.archlinux.org/packages/kwin-explicit-sync) also doesn't cause this issues.