It looks like some power state issue. The GPU runs as 700MHz (quite low) during daily use. When I started OBS it was changed to 1.4GHz, and the GNOME performance comes back to normal.
I didn’t find any ways to fix it in power settings with Wayland enabled. (if there is pls tell me) But nvidia-smi is able to modify the frequency of GPU, thus setting a lower bound for the GPU frequency.
For example, on my RTX2060.
sudo nvidia-smi -lgc 1395,2130
takes great effect.