I agree I see nothing bad on bjs side, the fact that cpu is more used when there is more activity / meshes.
Here is my test:
All of this is expected. Maybe you could reduce the load by not using PBR materials but this is not the root of your problem
Honestly, the only think I could think about is slowly removing features until you find what is causing the lag (that I do not feel in my case)
I do not see spikes: