Some customers noticed that since the move to ADE 5.1 (RockDB) certain backup jobs take more time than before.
An issue has been identified related to the behavior of the database when the data contains a lot of identical blocks (typically blocks with 0 data) as are often present on virtual disks.
A correction has been made and will be available in 5.1.9 ADE (September 2020) and an upcoming ATN 4.6 version.
As a workaround and or method to confirm the issue backups can be directed to standard HSS storage on the same server in that case you should create a dedicated set of tapedrives with a blocksize of 1M to insure the data deduplicates with the HVA storage.
To set the blocksize on a HSS drive go to the Drive properties and select Specific Variables the add the variable BS with value 1M.
Note when using parallelism you will need to have as many HSS drives as the number of backups in parallel.