Another possible issue is that the computer is using an older style "real" hard disc, and Windows 11 hammers hell out of traditional discs, probably because all the developers at Microsoft are using hardware that's based on Solid state discs, and they've not had to actually try to work with a machine that's got older disc on it.
That's very much the case with updates of the operating system, an old style disc can take many hours to do a full update, and in a lot of cases, one update is only the start of a chain of updates of other systems like .NET, with the result that the machine will be slow and sluggish for many more hours, even more so if it doesn't get to finish the update before it's powered down.
Even a Windows 10 updafe can take many hours on an older machine for the same reason, in order to restore some productivity to the use of these machines, they've been upgraded to SSD discs to reduce the time lost waiting for them to update.
The simplest solution if that's the case will be to seriously consider an upgrade of the disc drive to put a sensible size SSD into the machine, they're not massively pricey any more, and the difference in performance is chalk and cheese,
In Task Manager, as well as looking at the CPU utilisation, also check the disc usage, as that may well be a factor in the performance.
Are you using a third party anti virus product, as depending on the package, that can very much get in the way if an operating system upgrade is running, and on completion of the upgrade, it may well schedule a full scan of the system, which with some products can take a very long time to complete.
Another system killer can be if Microsoft search is set to index large parts of the disc, it may well be trying to regenerate the search indexes, which will also kill performance.
If it was easy to do, having endured over 30 years of nausea with Microsoft WIndows from the outset, when it was only a 13 floppy disc package, I'd be ditching Microsoft and moving onto an alternative, but there are problems moving some software onto non Microsoft platforms, and trying to deal with some organisations at state level is not easy if you don't use Microsoft products.
Windows now has become so bloated as time has gone on, it's reached the point where there are very few people that really know and understand what's going on internally with Windows, and the result is a package that's trying to be all things to all men, and it does none of it well any more.