EDIT 1: in my case, Google Drive was taking a lot of the CPU because I added 600'000+ (yes, more than six hundred thousand) files to my google folder in one time.
EDIT 2: after using it for about a year, I can safely say that I do NOT recommend using Google Drive. I am still regularly having issues synchronizing my files. Often the whole synchronization process simply stops & sometimes some files are re-added to my Google Drive folder. I might even have missed some other issues. I simply hope it did not delete any sensitive files.
How I fixed it:
Note: you might want to back up your file to an external drive before doing this.
On my computer: I zipped the relevant folders, and removed the unzipped version of these folders afterwards. From Google Drive online directly: I even removed the unzipped version of these folders via the Google Drive online web app, hoping it'd unload some work from the Google Drive app (not sure it whether it had a positive effect or not).
It then took a few tries to Google Drive to come back to a regular state but it seems like everything is good now (a few days later... after Google Drive app struggled to sync & even crashed a couple of times). Be patient, it might take it a long time to figure out "what the hell is going on".
Just got a 1TB Google Drive plan.
After uploading 140+ GB and working just fine, it is now going totally nuts: 88% of my CPU is being used... and it freezes my computer at some point.
I suspect it could be because I added a lot of file recently, from programming projects. Which means LOTS of files, all being quite small, some are extremely small.