Currently running backup exec 12.5 and 11d on two different servers. I was going to update 11d to 12.5 but wanted to resolve this issue first.
both running 2003
both use a scsi
both use a quantum sdlt600 tape drive for backing up

both are pulling sources from within(no network)
both start off about 1400mb\min
both throttle back to about 200mb\min

I have updated the drivers for the drives, made sure i have the latest patch for BE.

so I am at a loss for why they throttle back during backing up.