We have a Dell TL4000 with 2 x LTO6 drives. Most of our jobs are fine but we see lower speeds with a couple of our jobs. I've put it down fragmentation because of what ReFS is doing with blockclone.
We were running reverse incremental to make it easy to put a full backup to tape without any synthesizing but have since switched to forward with weekly synthetics just before we do our tape backup because reverse is even worse for fragmentation. It's helped somewhat but we still can't get sustained full speed on some jobs.
Our storage is a Dell 3860F with 60 x 4TB drives on 3 LUNS, each configured as RAID6 arrays (so the actual hardware isn't the problem)