Hi GKMan,
A little bit hard to work out your exact environment, as you list your OS as AIX, and NetBackup 7.1 or earlier, but then in the tags you have Windows Server 2003 and NetBackup 7.7. The last NetBackup version to support Win2K3 was 7.5 if memory serves me correctly.
Having said that, if you are on 7.7, are you using VMware policies to do the Exchange backup? I would highly recommend this, but there is not enough info about your environment - are the Exchange nodes physical or VM?
I can give you some information as to how we improved our performance with VMware Exchange GRT backups. My environment is NetBackup 7.6.0.4 Masters (running on Windows 2008 R2 64-bit) , with NetBackup 5230 Appliances as Media Servers. Exchange is 2013 running on Windows 2012 R2 64-bit.
We have two active DAG nodes at our production site, and one passive node at our DR site. We found similar to you, that when we backed up either of the active nodes, we would have performance issues including loss of connectivity to the witness server, and active users would lose connectivity in Outlook.
Due to this, we changed to backing up the passive node at the DR site instead. This eliminated those issues. Note that we have a witness server at each site. I'm not sure if you have the facility to create a passive node (or nodes), and do the same as above, but for us it was the solution. Note that my environment is much smaller, with only 4 DAG databases.
Hope this gives you some ideas,
Steve
