Exchange Mailbox Backups average around 2-4 gigs an hour. The limitation here is on the Messaging APIs that are used for the backups. The ammount of API calls required to backup at this granular level is where the bottle neck for this backup type is.
One recommendation is to create multiple subclients for the Exchange Mailbox backups. Each subclient makes its own connection to the Exchange Server and has the potential to run at about 2-4 gigs per subclient when ran simultaneously. If you go this route, I would usually recommend 3-5 subclients to start as each subclient will be launching a seperate backup process on the server with the Exchange Mailbox iDA installed.
CommVault Messaging Support