Hi guys, I just upgraded to the new OS Yosemite... and ever since then, my late 2009 iMac has been running extremely slowly and choppy, such as mission control. Not only that, but even doing simple tasks like saving/opening/closing a word document takes about 3 seconds for each task to complete. Opening new tabs in Finder takes another 2 seconds each... Generally speaking, the iMac was running very smoothly during Mavericks, but now it is slowed to a crawl with the new Yosemite update... I do not understand why is there such a big performance jump as my iMac lasted me very well from SL to Lion to ML to Mavericks no problem at all.. Even my late 2013 rmbp with 750m has very noticeable UI lag in every task it is doing. Anyways, just sharing this with you guys. I am not too sure if this update was ready for the public, and I hope this is fixed soon.