I checked for lab cases, and I didn't find any that reported on this behavior.
Did anything else change during the period of your upgrade to 8.1? I am pretty sure that, when doing a Object Copy (which is just a temporary Restore followed by a Backup), whatever was backed up to StoreOnce has to be 'rehydrated' first, which will slow thig down, but, from what you are saying, the only change has been your version of Data Protector.
Normally, when doing a Backup or an Object Copy to tape, we advise that you set a high concurrency, so I guess, that you should also check your Object Copy job by going to the Destination tab, expanding your device, get Properties for teh Tape Drive, and see what the concurrency is set to. I guess that it is a possibility that it may have been set back to teh default of 4 during the Upgrade, but, I couldn't explain how