Literally just looked this up out of curiousity https://www.phoronix.com/review/nvidia-windows11-ubuntu2304

and yeah, 5-15% seems "normal" but 20% is pretty reasonable considering all the other factors involved. But I would be concerned.

I would say to do the following:

1. make sure shaders have fully precached. Steam supports this in the background which makes me wonder if it is always finished if i start a game after an update
2. Check a few other games and especially engines. So Unreal, Unity, a few proprietary, etc
3. Look into using mangohud and other monitoring tools to try to see WHAT is different. Memory usage, draw time, etc.