I've got a 27'' iMac, late 2009 model. A few days ago, I suddenly experienced a slowness that is really extreme. I'm writing this from my MBA as the iMac has become unusable. For example: I'm trying to open system preferences right now, which is already taking a couple of minutes, I expect it to take 10 mins. Just for clearness: this is not a ''my five year old iMac has gotten slower, how can I speed it up'' kind of thing. Sure I noticed some aging but this happened ''overnight''. It's also not heavy use, just opening basic programs like Safari or Mail can take multiple minutes, and ain't usable after that. Specs: 27'', 1 TB, 3.06 GHz Core 2 Duo, 16 GB ram (upgraded myself, the cheap kind), running Yosemite (which was working fine before). What I've tried: - I obviously have enough RAM - I have enough free disk space (around 50 GB) - Did a SMC reset - Did a PRAM reset - also deleted some stuff according to second source, will try more but takes ages to do as well. atm I'm trying to edit start-up programs, also planning Verify Disk and reset printer system. Sources used, a.o.: http://machmachines.com/imac-running-slow-easy-common-fixes/ (aging iMacs kind of thing) https://discussions.apple.com/thread/3698221 (this seems to be my situation too) If anyone has any good ideas or theories, they're much appreciated!