Spinning beach ball woes on 27 inch iMac

Question

We have a mid 2010 27 inch iMac (and also a late 2010 13 inch MacBook Air, from which I am writing this post). About 6 months ago it was starting to get spinning beach balls, which I attributed to not having updated the OS (then 10.7). Having upgraded the Air to Mavericks with success, I also upgraded the iMac to Mavericks, and the beach balls got much, much worse, rendering the thing nearly unusable. I cannot for the life of me figure out what the issue is, particularly because the Air, which is about the same vintage, is working great under the same setup. Can someone who has not been staring at this problem for 6 months help?
Here's what I've got:
As I mentioned, mid 2010 27-inch iMac, 1TB hard drive, about half-full, backing up to a 2 TB Time Capsule.
We have two wifi networks to cover the whole house; one is integrated with the cable company's provided router and sits in the basement, and the other is the Time Capsule hooked up to the cable company's VoIP router upstairs. Because you cannot use the Time Capsule to extend an existing network if it is not an Apple network, I resorted to setting each wifi network up independently (separately connected to ethernet). My Air navigates this just fine, as do our iPads/iPhones, and I don't notice a speed difference between the two networks (assuming I have equally good reception wherever I am).
Notably, the Time Capsule seems to randomly stop broadcasting wifi every now and then (at intervals of a few days to a couple weeks), which requires a restart to get it working again. (This could be totally unrelated.)
The iMac was at 10.7 when the beach balls started; upgrading to Mavericks made the slowness/beachballs worse. As I noted, Mavericks did not cause any trouble with the Air.
The beach balls seem to coincide with a complete stall in everything the computer is doing. After the stall ends, the computer can be fairly speedy, so it's as if it pauses for several seconds to a minute or so to think very hard, and then goes back to its business.
There is usually a slight improvement if I shut down and boot up again, but it gums up pretty quickly.
Here's what I've done:
Started with 4GB RAM; added another 8 (total 12) after this all started. Equally bad before and after.
Stared at Activity Monitor and Console for long periods of time while watching for beach balls to try to detect a pattern.
In Activity Monitor, memory pressure is fine (esp. now that I added 8 GB), no swap used. The two worst offenders under CPU usage seem to be kernel_task or WindowServer, but they are not always monopolizing the CPU at the times when it is stalled.
In Console, the main two patterns seemed to be a frequent WindowServer "updates forcibly disabled" error, and what seemed like a constant browsing for wifi connections even though the computer was already connected to wifi. However, I couldn't figure out what either of these meant or what might be done to fix them.
Used disk utility, repaired permissions, verified disk, etc. - everything was ok, although some permissions were repaired.
Used third-party utilities, Onyx and iDefrag, to try to verify disk and clean things up. No significant problems detected, and no improvement.
Various other methods of cleaning up -- eliminating old software or files that aren't being used, emptying trash, taking extra files off the desktop, etc. In particular, having read that Mail might have issues handling Google accounts, I eliminated a heavily-used Google account from Mail and got a separate, lighter-weight third-party app to handle that account. That did seem improve things at first, but we seem to be back to slowness and beach balls again.
Final, last-ditch effort: backed up and then erased hard drive and (after another Disk Utility verify - still fine) did a clean install of Mavericks.
It took several attempts to download the installer using the Time Capsule network (kept stalling); switched to the other (non-Apple) wifi network and it downloaded and installed fine.
However, after the clean install, although it was not nearly as slow as before, amazingly, I still had spinning beach balls and hanging without having installed anything other than the OS.
I made several attempts to get into the backup to selectively reinstall files. Could not mount backup using Time Capsule -- option-clicking and selecting "Browse Other Backups" did not actually get me to the other backup. Using Migration Assistant, could not get the backup to load, or the loading was taking an inordinately long period of time, so I gave up. (Also, every time I tried to quit Migration Assistant to try to discover the problem with the backup loading, it hung and I had to hard-restart.)
So, since the clean install did not seem to fix the problem, and I was starting to worry about not being able to get to my backup, I have gone back to just restoring from the Time Capsule. It's telling me this will take over 200 hours.
The one thing I haven't tried is shutting down the computer, lugging it upstairs, and connecting it to Time Capsule via ethernet, at least for the restoration. I will do this if I leave the computer overnight and it's still not done in the morning.
I've done much searching of these discussion boards, and tried everything that seemed like a possible culprit. What am I missing? (Other than ditching this computer and buying a different one?)

Answer 1

We have a mid 2010 27 inch iMac (and also a late 2010 13 inch MacBook Air, from which I am writing this post). About 6 months ago it was starting to get spinning beach balls, which I attributed to not having updated the OS (then 10.7). Having upgraded the Air to Mavericks with success, I also upgraded the iMac to Mavericks, and the beach balls got much, much worse, rendering the thing nearly unusable. I cannot for the life of me figure out what the issue is, particularly because the Air, which is about the same vintage, is working great under the same setup. Can someone who has not been staring at this problem for 6 months help?
Here's what I've got:
As I mentioned, mid 2010 27-inch iMac, 1TB hard drive, about half-full, backing up to a 2 TB Time Capsule.
We have two wifi networks to cover the whole house; one is integrated with the cable company's provided router and sits in the basement, and the other is the Time Capsule hooked up to the cable company's VoIP router upstairs. Because you cannot use the Time Capsule to extend an existing network if it is not an Apple network, I resorted to setting each wifi network up independently (separately connected to ethernet). My Air navigates this just fine, as do our iPads/iPhones, and I don't notice a speed difference between the two networks (assuming I have equally good reception wherever I am).
Notably, the Time Capsule seems to randomly stop broadcasting wifi every now and then (at intervals of a few days to a couple weeks), which requires a restart to get it working again. (This could be totally unrelated.)
The iMac was at 10.7 when the beach balls started; upgrading to Mavericks made the slowness/beachballs worse. As I noted, Mavericks did not cause any trouble with the Air.
The beach balls seem to coincide with a complete stall in everything the computer is doing. After the stall ends, the computer can be fairly speedy, so it's as if it pauses for several seconds to a minute or so to think very hard, and then goes back to its business.
There is usually a slight improvement if I shut down and boot up again, but it gums up pretty quickly.
Here's what I've done:
Started with 4GB RAM; added another 8 (total 12) after this all started. Equally bad before and after.
Stared at Activity Monitor and Console for long periods of time while watching for beach balls to try to detect a pattern.
In Activity Monitor, memory pressure is fine (esp. now that I added 8 GB), no swap used. The two worst offenders under CPU usage seem to be kernel_task or WindowServer, but they are not always monopolizing the CPU at the times when it is stalled.
In Console, the main two patterns seemed to be a frequent WindowServer "updates forcibly disabled" error, and what seemed like a constant browsing for wifi connections even though the computer was already connected to wifi. However, I couldn't figure out what either of these meant or what might be done to fix them.
Used disk utility, repaired permissions, verified disk, etc. - everything was ok, although some permissions were repaired.
Used third-party utilities, Onyx and iDefrag, to try to verify disk and clean things up. No significant problems detected, and no improvement.
Various other methods of cleaning up -- eliminating old software or files that aren't being used, emptying trash, taking extra files off the desktop, etc. In particular, having read that Mail might have issues handling Google accounts, I eliminated a heavily-used Google account from Mail and got a separate, lighter-weight third-party app to handle that account. That did seem improve things at first, but we seem to be back to slowness and beach balls again.
Final, last-ditch effort: backed up and then erased hard drive and (after another Disk Utility verify - still fine) did a clean install of Mavericks.
It took several attempts to download the installer using the Time Capsule network (kept stalling); switched to the other (non-Apple) wifi network and it downloaded and installed fine.
However, after the clean install, although it was not nearly as slow as before, amazingly, I still had spinning beach balls and hanging without having installed anything other than the OS.
I made several attempts to get into the backup to selectively reinstall files. Could not mount backup using Time Capsule -- option-clicking and selecting "Browse Other Backups" did not actually get me to the other backup. Using Migration Assistant, could not get the backup to load, or the loading was taking an inordinately long period of time, so I gave up. (Also, every time I tried to quit Migration Assistant to try to discover the problem with the backup loading, it hung and I had to hard-restart.)
So, since the clean install did not seem to fix the problem, and I was starting to worry about not being able to get to my backup, I have gone back to just restoring from the Time Capsule. It's telling me this will take over 200 hours.
The one thing I haven't tried is shutting down the computer, lugging it upstairs, and connecting it to Time Capsule via ethernet, at least for the restoration. I will do this if I leave the computer overnight and it's still not done in the morning.
I've done much searching of these discussion boards, and tried everything that seemed like a possible culprit. What am I missing? (Other than ditching this computer and buying a different one?)