Exchange 2010 backup to tape takes too long
We have an Exchange 2010 server with 3 mailbox stores and one Public folder store.
I have four backup jobs that run everynight that perform full backup to tape of all those mailbox stores.
Mailbox store A = 233 mailboxes , 156GB
Mailbox store B = 117 mailboxes , 302GB
Mailbox store C = 224 mailboxes , 203GB
Public Folder = 212GB
For some reason the backup of store B takes about 4 hours on the weekend but takes 9-10 hours on the weekdays.
This store is backedup on 22:00 everyday and on 10:00 on Friday.
I noticed that when the job starts it's got great job rate of around 5000MB/min, but when I come back in the morning, the status is Updating Catalog and the job rate is around 500MB/min.
So I guess the problem is not with the Exchange server itself, it's in the Update Catalog phase.
The thing is that the backup of the other stores finishes quite quickly - Store A for example at around 1:30 hours on weekdays.
Even Store C which is the only one that is B2D takes around 4 Hours on weekdays.
Why is the Updating Catalog phase takes so long on this DB, and what can I do to improve it?
Is there a way I can see what was the job of the Backup process apart from the Update Catalog process?