The mailbox backup is inherently slow since each mailbox need to be logged
into.  1-2MB/s sounds about right.  Also, how heavily loaded is your server?
 There is a patch that addresses the corrupt object error, I can't recall the
number.   Make sure that the Subject line in the mail is less than 256 char,
otherwise the backups will fail.  Removing the mail would be your only option
if you want to avoid the errors.

paulg