I'm running on an old B+ I've had laying around for a couple years. Response seems quick enough but I only have a few modules on it. I did notice that when I tried to consolidate my Deluge, NAS, HomeGenie and Calibre servers all onto one Pi, HG got very sluggish. I've just rebuilt my HG all alone in it's own RPi and it's working fine. I'll rebuild the file servers on a separate Pi next.
As for difficulty, just get Jessie up and running, update/upgrade, get it set up as a headless unit. Then follow the instructions to install, noting the deb/tgz mistake. It took me awhile but that's because I wasn't being methodical enough considering how out of practice I am with linux.