Backup strategy for large mailbox stores

[ram]

We have cyrus servers deployed at many places where clients have varying
mail storage.
We have been taking backups to help in situations of  human errors
( where you get complaints like ..oops, I accidentaly deleted all my
mails!! )  and in case of hardware failures

Things have been working fine but off late we find that emailusage has
grown and so our backups take too long to complete .. we use dar to take
differential backups and take backups everynight. and transfer the
backup files to a remote server. 

If the backup is still running in the morning people notice a
considerable degradation of the server performance


Is there a better strategy , probably within the cyrus framework , to
take backups efficiently 


Thanks
Ram