I bought an iMac version 13,2 (basic configuration -- 1TB conventional hard drive, 8GB RAM) in early 2013. It came with 10.8 Mountain Lion and ran great. I even added 8GB of aftermarket RAM to bring it up to 16GB total. Then one day I upgraded to 10.10 Yosemite. The performance changed: Apps take a long time to load and run slower than before. For a while I accepted this (got busy with other things) but now I'm ready to back up all files and do a fresh install, hoping to restore my iMac to its former glory. Am I better off rolling back to an old version (like Mountain Lion or Mavericks) or should I upgrade to the latest and greatest (10.11 El Capitan)?