iMac extremely slow.

This iMac's problem is extreme slowness in response to command input.

I had started writing this query believing the problem to be a file system which would not boot. But gradually, going through the steps, with first finding that the machine would run Disk Utility, checking Disk Utility which returned no faults, then eventually to my surprise finding an apparently frozen boot progress screen had switched to a user selection screen, all did not seem lost, the problem now seeming to be the machine's speed.

It all seems to have begun when, on accepting an upgrade request from Firefox this 10 yr old iMac - which is I think on the final available software - froze with normal display and continuous action rainbow whirly, so I attempted a Firefox normal Quit, then Force Quit. After a long wait with no apparent progress, I tried Apple/Restart. This eventually reported Firefox not closing, so Firefox Force Quit attempted from the report which must have worked as, after this non-aggressive closure of a couple of other apps followed automatically and the machine went into restart. It progressed very slowly through the bar and I don't recall whether it went gracefully into another Apple and bar or merely went to the end where it stayed again. I tried a POR which eventually came to the same apple with progress bar at the end.

After another long wait the same was tried again with the same response, so after several repeats a CMD R POR was done, which amazingly succeeded in bringing up Disk Utility.

The installed disk is a 1TB original. Disk Utility reports the following:

Macintosh HD

APFS Volume - APFS

macOS 10.15.7. Then over on the top rhs, under a 1TB label is indicated 'SHARED BY 5 VOLUMES'.

However, at the top LHS Disk Utility shows:

Internal

Macintosh HD

Macintosh HD - Data

Disk Images

macOS Base System

First Aid was done on the Macintosh HD - Data, which reported no errors, exiting normally. Then on macintosh HD, which came also back normally. So then First Aid done on all items again including the Disk Image. Each came back normally.

So Apple/Restart done, which brought up the Apple and Bar, running to the end and staying there. After a long wait, still no change so POR done with same result, and again with CMD R to bring up Disk Utility again to confirm the above details, and confirming what I'd already known about a third of the 1TB is unused.

I tried an Apple/Restart from Disk Utility screen. Same results as previously.

But whilst compiling this script, the iMac screen very surprisingly changed to the User selection screen. I selected my usual login, which after 10 seconds blanked the screen, and after a further several minutes, my login's wallpaper photo came up with a working rainbow whirly, which after many minutes is still whirling, and going by how long it took to change the previous displays, will take many more to settle. It has just now - almost - with some image icons displays still to be completed.

It seems that rather than failing to boot properly, my iMac is now just extremely slow, much slower in fact than it used to be prior to linking it, my Macbook Air and iPhone all through iCloud about 4 - 6 months ago, but slower even than it was yesterday, or previous several weeks which was just about usable, if annoyingly hesitant.