Ever since I installed Yosemite on my iMac, it's become unresponsive for about 5–10 seconds when I do any of a variety of common actions. The following usually trigger a delay: a few seconds of no response at all, followed by a spinning beachball for 5 seconds, and then the action happens:

  • Scrolling a web page
  • Moving the mouse cursor over a button on a web page
  • Typing text into a field on a web page
  • Switching from one app to another

I find this extremely jarring, to the point where the computer is almost unusable. I've tried a whole lot of Yosemite speed-ups found on web pages, and while they did improve the general sluggishness of Yosemite, I haven't yet found a way to stop these sudden "seizures" that it seems to experience all the time.

My question is: What is Yosemite doing when it becomes unresponsive in those situations? Or, better yet, what's something I could do to find out? I see a lot of question-and-answer posts on the Internet suggesting fixes to Yosemite's slowdowns, which often don't fix the specific problem with a user's machine. I'd like to know how to diagnose the problem, separately from prescribing a treatment for it.

Watching with top and MenuMeters, I see that the CPU capacity is mostly unused. (MenuMeters continues to update during the spinning beachball.) There's also usually plenty of free memory (911 Mb when I look at it right now). So, it appears that Yosemite's "seizures" don't result from exhausting system resources. It doesn't appear to be memory-swapping. What, then, is it doing—or how could I find out?

Yosemite version: 10.10.2
iMac: 21.5 inch, Late 2009
Processor: 3.06 GHz Intel Core 2 Duo
Memory: 4 Gb 1067 MHz DDR3
Graphics: NVIDIA GeForce 9400 256 Mb

  • I'd look at memory pressure & swap used in Activity Monitor - with only 4GB RAM & presumably an HD rather than SSD, it could just be heavy paging. I found Mavericks was the start of my similar issues, which I eventually cured with a whole lot more RAM & an SSD. – Tetsujin Sep 20 '15 at 13:43
  • Mine has 8GB and frequently does the same thing - for example when dragging an image from Safari to a folder on the desktop – MicroMachine Sep 21 '15 at 4:36
  • Sounds like iowait. – bot47 Sep 21 '15 at 18:23

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