INOPERABLE IMAC AFTER UPGRADE SOLVED!!!!! UNCHECK ALL ITEMS ON SPOTLIGHT PREFERENCES.
Well, past weeks have been a nightmare after upgrading to El Capitan. I have a 20111 imac 27", it has gone from super fast to basically inoperable, even running Clean My Mac 3 every half an hour...memory usage was going from 12Gb to 1 in a matter of clicks, everything would freeze, even icons would not show anymore, thats how slow it had gone. Last night reading this forum, I decided to give it a go to some suggestions, I reset the SMC by umplogging it from powers for a few seconds, and ran a full scan with Clean My Mac, restarted, seemed slightly faster. This morning I went on system preferences/spotlight and unchecked everything, why do I need this bloody thing indexing files anyway? Cant I remember where they are? so...surprise surprise, my mac is super fast again after unchecking every single item on spotlight settings. This may help everyone out there in need for urgent help like i was.
Good luck.