If you see extremely high CPU usage by box sync on MacOSX (Mountain Lion), one of the reasons could be locked files. For some reason, a large number of files in my box sync folder were marked unchangeable on the filesystem. This would cause box sync to repeatedly try and fail in uploading these files.
To fix this, look for such files and unset this flag:
$ cd /path/to/box/sync/folder
$ find ./ -flags +uchg -print0 | xargs -0 chflags nouchg