Ok, well there are a couple of suspicous lines:
PrefersNonDefaultGPU=true
X-KDE-RunOnDiscreteGpu=true
EDIT:
I believe that PrefersNonDefaultGPU=true forces steam to use the integrated GPU.
I’ve been using envycontrol to setup my environments to exclusively use the discrete GPU. Apparently, this didn’t work on Wayland which is probably still using hybrid mode. That could explain why steam.desktop worked over there and also why it feels less responsive compared to x11.