Dropbox (and other apps) has a tendency to use every bit of CPU it can, especially when starting up. On Mac (with Homebrew installed), Dropbox (and other apps) can be throttled via a tool called cpulimit.
brew install cpulimit
Get Dropbox’s Process ID:
ps aux | grep Dropbox user 19628 104.7 2.3 xxxxxxxx xxxxxxxx ?? R 4:07PM 15:47.12 /Applications/Dropbox.app/Contents/MacOS/Dropbox
Run cpulimit with the -p flag using the process ID:
cpulimit -p 19628 -l 40
Note: That’s a lowercase L (for limit) and the last number (40 in this case) is the percentage % of CPU you’d like to throttle the app to.
Also Note: cpulimit will output ‘Process xxx found’ and will continue to run until you kill it with <CTRL> + C