If I unzip a large archive in /tmp, Dropbox is eating 60% of my CPU.
If I open the new Xcode for the first time (and the system verifies all the signatures) Dropbox is eating 100% of one CPU.
It really seems like the Dropbox client is monitoring the entire filesystem (all FSEvents) instead of just the dropbox syncing folders, and doing it relatively inefficiently at that.
At this point if I'm doing anything filesystem intensive I close Dropbox first.