Ok, I'm going to man up and make a confession: I just realized that the disk space on the Drobo was getting low. From the Drobo website: "Instead of refusing to copy files when its capacity is almost full (over 95 percent full), Drobo devices instead enter a slow-down mode."
I freed up space and things are now running smoothly.
I am having similar issues though my Drobo is about 80% free space 5TB out of 6.
My iTunes library is there and I also use it for my Time Machine backups. As a result, whenever time machine is doing its thing (every hour) iTunes becomes almost unusable. Just scrolling through the list of apps to select/deselect on my phone becomes excruciating as every scrolling movement on the magic mouse results in a 20 sec beachball.
Apple TV seems to play fine regardless.
As to time machine, the last hourly backup (of about 58MB of data) took more than 20 min - so one third of the hour before the next one.
Yeah... since i upgraded my drobo to the 1.3.7 firmware, its become the slowest device in my system. And its connected via the firewire 800 port. My USB2 2.5" drives absolutely kill it for read write speed.
Just waiting for the thunderbolt arrays to come out, or in the case of the Promise Pegasus unit Apple have for sale, a reasonable price to be attached to it. The Drobo will then become the offsite backup unit.