For the past week, my iMac (Retina 5K, 27-inch, Late 2015) has been unusably slow if it's left unused for too long. I selected "Prevent your Mac from automatically sleeping when the display is off", which seems to have helped a bit, but when I left the machine for a couple hours earlier today I had the same problems again.

What I get is either:
1) If waking from sleep sometimes it would crash and reboot after I enter my password, with the error message being Window Server not responding for 120 second
2) More generally, and since disabling sleep, what I get is unusably slow performance. Every click creates a beach ball, but eventually most will actually produce a respond after 60 seconds plus. The iMac generally stays responsive enough for me to be able to trigger a software reboot, but the most recent time it didn't.

I have a Fusion drive, and as this is my third iMac with a Fusion drive my suspicion is it might be to blame as that's usually what goes.

Disk Utility has found no errors. I booted in to Apple Diagnostics mode and that also found no errors.

I'll probably try a wipe and reinstall next but am worried that if the root problem is hardware I might end up in a worse state.

Anyone seen similar problems and can offer advice?

I have a suspicion that APFS slowdowns Macs with Fusion Drive or HDD.
Thinking that reinstall would help for some time because it will remove the filesystem.

You can test it by installing older macOS that utilizes Mac OS Extended rather APFS as a default file system (like High Sierra)

Personally, I have never been a fan of the whole "Fusion Drive" concept. The obvious benefit is that you improve the overall performance of the system with a small SSD alongside your spinning rust disk... but the disadvantage is that now you have two components which can potentially fail over time, where before you had only one. And of course, the failure of a drive is never immediately diagnosable; in my experience it has to already be really close to the point of total failure before the software diagnostics will start complaining about bad sectors and what-have-you. By that point, you've almost certainly already lost data, even if you don't know it yet.

I'd say get that Fusion Drive out of there, and replace it with a good sized SSD. They're much cheaper these days, so there's really no good argument for bothering with Fusion. Regardless of whether the cause is APFS or one of the two drive mechanisms failing, I think you'll definitely be better off with the single SSD in the long run.

Grymbok,

Download Drive DX, which is free to try, and see if your drives are in good condition.
Here's the link: https://binaryfruit.com/drivedx
You can set it up so you'll have an icon for DriveDX in your menu bar. It'll have a green dot if all is well.
Excellent application for checking the health of your drives. Open the app and check out the stats on your two drives.


By the way, I also have a late 2015 iMac, 250GB SSD, that is my production machine. It's running quite well and I plan on continuing to use it for another couple of years.

Monterey is the last Mac OS that will work with your computer. I mention this because security updates will only be available for another 2 years once Mac OS 13, Ventura, drops this fall. Once the security updates stop for the late 2015 iMac I'll be retiring it, and replacing it, hopefully, with a new iMac Pro M3.

stradify said:

Grymbok,

Download Drive DX, which is free to try, and see if your drives are in good condition.
Here's the link: https://binaryfruit.com/drivedx
You can set it up so you'll have an icon for DriveDX in your menu bar. It'll have a green dot if all is well.
Excellent application for checking the health of your drives. Open the app and check out the stats on your two drives.


By the way, I also have a late 2015 iMac, 250GB SSD, that is my production machine. It's running quite well and I plan on continuing to use it for another couple of years.

Monterey is the last Mac OS that will work with your computer. I mention this because security updates will only be available for another 2 years once Mac OS 13, Ventura, drops this fall. Once the security updates stop for the late 2015 iMac I'll be retiring it, and replacing it, hopefully, with a new iMac Pro M3.

Click to expand...

Thanks, I’ll try that.

I’d missed that announcement about Ventura. Good to know. I’ve been thinking about upgrading but I’d rather not have the expense this year if I can avoid it, especially given the lack of a clear upgrade path for those of us used to a 27” screen.

zarmanto said:

Personally, I have never been a fan of the whole "Fusion Drive" concept. The obvious benefit is that you improve the overall performance of the system with a small SSD alongside your spinning rust disk... but the disadvantage is that now you have two components which can potentially fail over time, where before you had only one. And of course, the failure of a drive is never immediately diagnosable; in my experience it has to already be really close to the point of total failure before the software diagnostics will start complaining about bad sectors and what-have-you. By that point, you've almost certainly already lost data, even if you don't know it yet.

I'd say get that Fusion Drive out of there, and replace it with a good sized SSD. They're much cheaper these days, so there's really no good argument for bothering with Fusion. Regardless of whether the cause is APFS or one of the two drive mechanisms failing, I think you'll definitely be better off with the single SSD in the long run.

Click to expand...

As @stradify noted, this iMac is coming to the end of its supported life, so I’m not likely to start trying to modify the hardware at this point! Thanks for your thoughts though.