There have been a few reports of Macs running recent versions of macOS, particularly 12.0.1, gradually getting slower until they almost grind to a halt. This article suggests a structured way to tackle both diagnosing and dealing with this.

Before you bring your Mac back to life, if possible spare a couple of moments to record some vital clues as to what has gone wrong.

Apple does provide one way of capturing full details using sysdiagnose, which is essential if you’re going to report the problem to its engineers. You invoke this by pressing the Shift-Control-Option-Command and . (period or full stop) keys together, after which the screen should flash once, then several minutes later a window opens showing where the diagnostic dump file has been saved, so you can carry on.

The snag is that, unless you are an Apple engineer and know your way around the log files and other data captured, this is of no value to you. It’s also easily captured using the command in Activity Monitor’s View menu.

The two most useful things you can do now are to write detailed notes recording what happened, and to study the panes in Activity Monitor. The two most important panes at this stage are CPU and Memory. Check first that Activity Monitor is set to show all processes in its View menu, or it may conceal some of the most significant. In the CPU tab, click in the % CPU column header to sort them with the highest at the top, and in Memory order them using the Memory column.

At least take a screenshot of each view for future study, even if you haven’t time or inclination to do that now. Sometimes, the Disk pane can also be worth a check, as can Network, but the big two are CPU and Memory, looking for apps or processes which are taking more than around 10% of CPU or more than about 4 GB. Of course, some usually do anyway, but all the heavy users need to be accounted for in your process to eliminate causes.

There’s little point in opening Console to look at what’s going on in the log, sadly, unless you’re already adept at its use and know how to interpret what you might see there. Even if you do think looking through log might help, that’s probably best done retrospectively, using a browser such as my free Ulbow, once your Mac is running sweetly again.

Having gathered the most important evidence, it’s time to recover your Mac.

If you’ve been able to identify an app or process which appears to be the cause, you can now try quitting it, to see if that restores normal performance. Try these in order of preference:

1. Bring the app to the front, and quit it normally after saving any open documents, if that still works.
2. Open the Force Quit dialog using Command-Option-Escape, select the app there, and click the Force Quit button. Although this should be gentle force, any unsaved documents are likely to lose their recent changes.
3. In Activity Monitor, select the app or process and press Command-Option-Q. This will definitely lose any unsaved changes in documents. Never try doing this with kernel_task, though, which is using CPU to protect your Mac.
4. If you know what you’re doing in Terminal, you can kill the app or process there if you prefer.

If that isn’t appropriate or doesn’t work, the next least disruptive thing to try is closing all your apps, saving what you can on the way, then logging out and back in again. For some types of problem this is usually the best solution – WindowServer is a good example, as that can only be restarted by logging the current user out.

Next in line is to simply restart your Mac. This may be a first choice if, like me, you normally leave it running (even if asleep) for several days at a stretch. There was a time when most Mac notebooks didn’t like repeated work-sleep cycles, and were much happier if every few days they were shut down instead of being put to sleep.

If your Mac is still sluggish and unresponsive after restarting, try a Safe boot, which flushes many of its caches and can clean up temporary files and other junk. While this is simple on an Intel Mac, where it’s initiated by holding the Shift key during startup, M1 series Macs can only enter Safe mode from Recovery, so you’ll need to shut down, then start up with the Power button held to load Options. Once the Mac has started up and is offering one or more disks, select the disk which you wish to boot from in Safe Mode, then press and hold the Shift key and click Continue in Safe Mode underneath it. If you try the normal Shift key technique on an M1 series Mac, it will just start up normally.

Beyond that, you’re into normal troubleshooting procedures. There are two important points to note:

• If you’ve recently upgraded from an older version of macOS and your Mac is starting up from a hard disk, disk performance can deteriorate to the point where it affects everything your Mac does. You’ll be much better off booting it from an external SSD if you can.
• If your Mac is running Big Sur or Monterey, there’s little or no point in reinstalling macOS. In those two versions, the fact that your Mac boots up normally is a guarantee that its files installed on the System disk are in perfect condition, just as Apple intends. There still could be plenty of problems lurking on your Data volume, though, but they’re best tackled by identifying them and not simply reinstalling.

I wish you success, and to your Mac, a swift and full recovery.