Perhaps you haven't looked? Try cloning a git repo, or unpacking a tarball with many files (doesn't matter if it's inside Dropbox or not) and observe the CPU usage of the Dropbox process.
I checked right now and the Dropbox process on my machine has consumed a total of 39 minutes CPU time. That's 39 minutes of full-scale CPU usage. Even their "web helper" process (whatever that is) consumed two minutes and 41 seconds.
(Google) Backup & Sync currently shows 2:04.46 in CPU time on my Mac. I'm guessing that means 2 hours and 4 minutes. The machine was shutdown overnight and booted this morning.
Is this something that resets with each boot?
Sounds broken on user end. Never seen anything like this when using Dropbox on several versions of Windows and Ubuntu, on both low-end and high-end machines.
I checked right now and the Dropbox process on my machine has consumed a total of 39 minutes CPU time. That's 39 minutes of full-scale CPU usage. Even their "web helper" process (whatever that is) consumed two minutes and 41 seconds.