That's because IntelliJ is creating an (I think) inverted index and dependency graph of your project for code completion/Intellisense and it uses multiple CPU cores to do this as fast as possible.

VSC won't do this to the same extent.

Inverted indexes are slow to create, but fast to query.

VSC != VS

I'm sure VS chugs power like IntelliJ too

Similarly, it's a pain because I found IntelliJ was far better at auto-completing modules across a javascript mono-repo, but the performance was bad that I've switched to VSC despite it generally being worse experience