I have a mid 2011 base iMac 21.5" 500gb HD which runs slow - I get alot of beachballs.
I'm having the same problem with the same machine. I haven't added anything to my iMac - it's a stock standard mid-2011 base iMac 21.5". I've only had the iMac since January 2012. I've noticed over the last few days that the machine is super sluggish, especially when using a browser. I primarily use Firefox (which is up to date), but using Safari gives me the same results - incredibly slow, lots of beach balls and it takes five minutes or more to load the simplest sites. Even when it does load sites, it hates me scrolling down the page and will launch beach balls regularly and seemingly at random. It seems to hate Twitter the most, which is unfortunate because part of my job is maintaining a daily social media presence.
My iMac specs:
Processor 2.5 GHz Intel Core i5
Memory 4 GB 1333 MHz DDR3
Graphics AMD Radeon HD 6750M 512 MB
Software Mac OS X Lion 10.7.4
I currently have 413.84GB free on my harddrive out of the available 499.25GB. I also have a 1TB WD Passport external HD attached via USB to the computer as a main backup drive. It has over 800GB left on it. I use a bluetooth mac keyboard and magic trackpad.
I recently updated to 10.7.4 - wondering if it could be some issue with that update? This is my main work machine, so I'm very eager to get this sorted out. If I thought it needed repair, I'd book it in post haste. (I have AppleCare on this machine, thank goodness.)
Any suggestions as to what the problem is would be appreciated! Thanks.