That's probably because 8GB is not enough to run Intellij comfortably and the system starts swapping things in and out of memory constantly. Another thing that happens when intellij uses up its heap is garbage collects including frequent stop the world varieties of that. The solution is to configure it to have more heap. Either way, that's a memory problem and not a CPU problem.

8GB for a development laptop is not a great idea in 2021. I've been using 16GB since 2013. I'll get more with my next laptop, which may or may not end up being the 16" M2?