Hm, could still be the backup thing; IIRC they take a couple minutes to finish, but after that Dropbox will start uploading it, and I think I saw it use quite a bit of CPU time on occasion. IDK.
I should set up a home server of sorts, have it download the weekly backup and slow-upload it to Amazon Glacier, a low-cost long-term storage thing. I've got a full backup of my persona shit (pictures, documents, nothing sensitive I think) on there for $0.44 a month,
. The main problem is that uploading shit takes forever. As does compressing it into convenient archives, for that matter.