Maybe this is heavily dependant on the file structure of what's being synced, but I've tried Google Drive, iCloud, OneDrive, and Dropbox, and all but Dropbox have exhibited the CPU melting issue you describe.
Dropbox was the only one that managed to sync 10s of gigs in a timely manner, without spending hours "analyzing files" before starting to upload.
Dropbox was the only one that managed to sync 10s of gigs in a timely manner, without spending hours "analyzing files" before starting to upload.