I have a mid 2010 mac mini - 2.66 GHz Core 2 Duo 4 GB Ram. Using it for home media server and misc apps. It was running Mt Lion or Mavericks (I can't recall) and I updated to El Capitan. It got ugly. Seems like I have to reboot nearly daily to keep it running "smooth" and even then its laggy. I'll be happy to do a fresh install of a prior OSX but not sure what the experts recommend? Also, Can I just do a USB clean install from Mavericks or Yosemite download from the App Store or do I need to have proof of prior version? (I think I have a Snow Leopard disc).