Maybe check this thread: Desktop Lag/Stuttering while gaming - #2 by bogdancovaciu
Please check also this file: /etc/profile.d/kwin.sh
Maybe this is needed to add?
echo "export KWIN_TRIPLE_BUFFER=1" | sudo tee /etc/profile.d/kwin.sh
Sorry, I am not an KDE-User. So no idea. But it 100% related to opengl and the gpu.