I think this is it, I was able to get through several YouTube videos and Helldivers is back to getting my max frame rate on Proton GE. Thank you for doing god’s work cause I was wracking my brain for 2 days. Issue is a bug with KWin 6.0.4-1
I will wait until it’s been released in the repos before I update my system again.
The fix per DyingKino on Reddit:
https://www.reddit.com/r/linux_gaming/comments/1c835fp/arch_kde_crashes_when_fullscreen/
It’s most likely this bug introduced in 6.0.4: 485730 – Severe stuttering with Plasma 6.0.4
There’s also already a fix in: scene/workspacescene: don't check direct scanout candidates for a pixmap (!5626) · Merge requests · Plasma / KWin · GitLab
You can downgrade to 6.0.3 by setting Server=https://archive.archlinux.org/repos/2024/04/16/$repo/os/$arch in /etc/pacman.d/mirrorlist and running sudo pacman -Syyuu.
Note: you have to put that Server URL at the top under all the comments for it to work. It will downgrade all packages to the time of the 16th.