Extremely high cpu usage when using nix-shell with cuPy

It does look like cuPy runs on the integrated GPU (Intel) instead of your dedicated GPU (Nvidia).

hardware acceleration needs to be enabled / set for the Nvidia GPU.
Maybe have a look at that post: Nvidia open breaks hardware acceleration - #3 by TLATER

You prolly can try to run nvidia-smi when running your python project to check if it’s running on the Nvidia GPU and confirm your issue.