thanks for sharing info;
I am not that good at Storages.
Anyways, as a quick check, if you have available required space; can you switch the backup of each job to other's backup drive just to check, whether the time difference is because of storage performance or because of some load on databases?
Secondly, it is good to have storages attached locally to get good performance on backups;
Lastly, if you can have multiple drives for each backup job; so backup can get be written almost at twice speed on two drives as compare to writing to a single drive.
One more thing; in backup jobs; there is an option of MAXTRANSFERSIZE; ideally this should not be specified to get maximum backup performance; so just to be sure, check this setting as well in your backup script.