I was working with a Blender sculpt tutorial and suddenly my system slowed to a crawl with the “beachball” spinning and the program locking up. Yesterday I removed that .blend file from the disc, but I am noticing the spinning beachball sort of flashing off & on still and things still lagging. I haven’t had this problem until now. I have an IMac 27 (late 2009) - 2.66GHz Intel Core i5 (quad core) - ATI Radeon HD4850 with 512MB of GDDR3 memory. I know a little about IMac’s and computers in general, so if there’s someone who can offer some help, please understand if haven’t provided you with enough info. Thanks!
When that happens to me, I pull up the Activity Monitor and sort by the CPU usage to see what is causing the slowdown. If it is an app I am not using, I force it to quit. Also, I check memory usage. If an app is using massive amounts of memory, depending on what it is, I’ll force quit that too. And then things go back to normal.
Thank you MichelWeber! I will try this and let you know how it works out. 
Well, I checked the Activity Monitor like you suggested but nothing was using up CPU%. I opened Blender and did some basic modeling with no materials added and I was still seeing that spinning ball flash on intermittently. I’m just wondering if anyone else is experiencing the lagging issue?
There is always something using CPU% - do you mean there wasn’t anything using a majority of the percentage? What about memory? If you order the list by memory usage what is at the top and how much is it using? If it is using something over 1000 MB, that might be the cause of the beach-ball.