How important is to have a decent GPU for smooth desktop experience currently and in near future? I was considering switching to less power hungry GPU, but I fear it might make DE slower.
Important are drivers. Integrated intel or AMD GPUs are fine. Also KWin has CPU software rendering, that works mostly fine.
GPUs have gotten to the state that anything not bottom of the barrel can do a composited desktop just fine. It's mostly ARM chips with anemic GPUs that can't do it. I'm looking at you, Raspberry Pi.