can you notice any changes taking place via dashboard, for example for the used space or free space?
i do know that on a das drobo, changes to files, or in this case, file or folder deletions, can complete more quickly from the computers perspective, but then actually require some time for the drobo to run through is space reclamation process, to essentially free up the blocks again.
im not sure about the mac, but maybe that backup file, being huge, will just need a longer time?
(if you can see the free space increasing bit by bit, every 5minutes or so, then maybe that bug file is just presented to the operating system as a big file, but might be made up of lots of files a bit like an iso file, and is being deleted in parts?)
btw, what happens if you find a spare, un-needed small file somewhere else, such as a 1MB file - does that 1MB file get deleted relatively quickly?[hr]
also, can i check what your main used and free percentages are on the drobo?
(if things were quite full, and you already had a solid red light and near or passed the 95% used mark, that might also account for slowdowns)