You may want to refer to the EV Performance Guide.
FROM PERFORMANCE GUIDE
When archiving, the Archiving Service takes items from the Exchange Server and puts them onto the MSMQ queue "enterprise vault storage archive". In order to prevent an excessive number of items from being put on the queue, a limit is set to the number of items. When this limit is exceeded, the archiving tasks will pause to give the Storage Archive processes time to clear the queue. By default, this limit is set at 1,500.
The above is from section: 'Exchange Mailbox Archiving' which also covers journaling. There are some parameters that can be tweaked depending on your system configuration. If you system is processing items faster then it can store them then the PIPE to the storage may need to be opened.
You will see what I mean when you read the section.
Jim S.