I have a Drobo FS with 5 2-TB drives and I’m seeing very slow file access performance. Once a read or write begins, the transfer rate is fine, but “statting” files (doing a directory listing, calculating directory size, etc.) is very, very slow. Getting Finder to display a medium-size directory can take 30 seconds or more, with the spinning beach ball throughout.
Also, mounting a volume can take forever. From the time that I click on the server name in Finder to when I see the list of available shares can be 30-60 seconds.
And deleting files is pretty much a non-starter. I just SSH in and use rm.
I’m running AFS, 1 GB networking with a Dell enterprise-grade switch, and 1 or 2 Mac computers running OS X 10.6.5. My MTU is 1500, since that seems to be all that the MacBook Pro supports.