IDE's a notoriously expensive to run. As soon as you open VS Code it loads the program from your hard drive, into RAM; acting as a staging point for all the processes VS Code uses to manage its environment. Things like,
- Overhead of the Electron framework upon which it is built
- Checking for external file changes that need to be synchronized to the editor
- Render pipeline
- Child processes to support any extensions you have running
- Terminal instances (and by extension anything running in those terminals)
Here's a nifty little extension I found after some quick Googling. It will show you the subprocesses running in VS Code, and may help you identify exactly what is taking up the most bandwidth. Do keep in mind, that by killing some of those processes, you may begin to lose the associated functionality, and indeed possibly even cause VS Code to crash. The only sure-fire way to keep it from taxing your CPU, is to shut it down completely when you're not using it.
Perhaps you could try out another IDE like Sublime, IntelliJ, or Atom and see if they act more as you expect when idle. Personally, I really love the features of Jetbrain's IntelliJ (and similar: Webstorm, PhpStorm, etc).