We are having the same issues. Background: We stood up a new server and consolidated three Exchange servers (2007) into one running 2013, so we migrated users to the new server (not an in-place upgrade) and retire the old. Total database is just under 300 GB, compared to 200 GB previously for the server I had to back up of the three geo-distributed servers. I had my jobs initially situated the same as we did with the 2007 server, to do backup and cataloging immediately following for GRT but the cataloging took forver by comparison to 2007 (30 minutes vs. 6+ hours). The support group via ticket moved us to Instant GRT, which improved the backup to disk to a little over an hour (6,000 MB/minute) since the cataloging was done "instantly" and not as a separate job. That was beautiful.
The problem was that this caused duplication to tape to now take forever. It is the same kind of scenario. It initially moves the data to tape in about 2 hours (3,000 MB/minute) and then hangs, appearing as if it is doing nothing. It will complete about 5 to 7 hours later. The verify step on the job takes maybe 20 to 30 minutes, but doesn't kick off until it finishes the backup, which has hung up for 5 to 7 hours. For whatever reason the ticket we have has support hung up on the overall job rate, not the fact that the data appears to copy very quickly and then get hung up, which causes the job rate to diminish slowly over the apparently idle time.
If I actually try to duplicate incremental jobs to the tape along with the full, the job runs in excess of over 17 hours for maybe 350 GB of data. It runs too long so I end up cancelling it since I have other jobs I need to get running.
For now I just have to schedule the full backup to be duplicated during long idle windows for other jobs. We do not do any deduplication or anything fancy. It is just local hard drive storage and a single bay LTO5 tape loader library. The backup job isn't doing anything but the Exchange information store.
If we find resolution with the helpdesk (ticket is three weeks old now) I will post it here in case it helps anyone else.