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:
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
Check a few other games and especially engines. So Unreal, Unity, a few proprietary, etc
Look into using mangohud and other monitoring tools to try to see WHAT is different. Memory usage, draw time, etc.