Using thie backup destination for accounts with large amount of files is not recommended. Due to DropBox API limitations, there is too much overhead for each file, which makes file transfers very slow.
We recommend using this for full compressed backups only.
JetBackup currently doesn't support compressed backups. We only copy files, without compressing them.
You can however use cPanel's internal backup to create the compressed backups to a local folder, and then use JetBackup to push these backups to the remote dropbox folder.