Can you suggest a (series of) test(s) or monitoring that can show what is happening when the system fails, or some test that can show that the harddisk is borked or if some network issue is at hand?
Upgraded from Mountain Lion. For a week or so it was like my box was on steroids, then I saw the beachballs more and more often. Took a backup, formatted my disk and re-installed Yosemite from scratch - no improvement.
Last two times I started the box, Safari, which I just ran to save a wallpaper is stuck with a great big spinner. Nothing else can be run and even the top bar was missing. When I turned the computer off and on, I was sent to System utilities which made me suspect a harddisk failure is imminent and perhaps the root problem. I have now let the computer sort itself out - when I woke it, all tabs in Chrome were in a state of loading, but immediately got the pages loaded and I have for now as I write this a very responsive system.
When in Chrome (my default browser, I sometimes get no response, then a beachball, then more often than not an "Aw Snap" or Kill page? which make me suspect a network issue (all other devices are happy with my time capsule - iPhone apple TV and iPad - yes I am 100% Apple here)
I contacted Apple Support with this story as a Yosemite issue and was told to re-install snowleopard which came with the machine. I want to make sure my iMac is borked first since then I will upgrade to a newer iMac instead.
The fact that I am now breezing along with no issues and pages loading and switching much faster than under Mountain Lion makes me want to first investigate if there is something you guys can detect based on some test you can suggest.
Model Name: iMac Model Identifier: iMac8,1 Processor Name: Intel Core 2 Duo Processor Speed: 2,66 GHz Number of Processors: 1 Total Number of Cores: 2 L2 Cache: 6 MB Memory: 4 GB Bus Speed: 1,07 GHz Boot ROM Version: IM81.00C1.B00 SMC Version (system): 1.29f1
UPDATE If your iMac slows down and hangs like mine, have a look in the console for hardware issues. An SSD in addition to the 4GB I already had resulted in a fast iMac.