iMac slows down after Big Sur installing

Thank you erikxy! Removing Avast fixed it for me. Computer is now running fine with Big Sur. Hope this works for others as well!

Big Sur would not install problem free on my iMac 27 2017 (3.4 GHz Quadcore I5/1TB fusion/24GB). It hung halfway in a reboot loop, repeatedly giving the multi-language error message that something went wrong, wait for restart. After an hour I pressed shift, after which it continued and finished the installation and started Big Sur in safe mode. Performance was impossibly slow, peripherals could not be seen. Rebooting resulted in same error - iMac could only be started in safe mode, and slow performance prevented any practical use. Catalina had to be re-installed. Reverting to Catalina was also tough because the Time Machine restore would not boot, a clean Catalina install was required also. Next to the iMac on my desk I have a 2.4 GHz 2014 Toshiba laptop with SSD/16GB and Windows10 on a 27" UHD monitor, runs great - MacOS is beginning to be seriously less hassle free than Windows.

@Lostinlodos 

I just created a new user and gave that user admin rights. For the most part it's easy enough to migrate but if you have licensed software you now need to enter the license again

For me the hard part is that I have a clients microsoft remote desktop setup and that can't be migrated. Which means I'll have to ask the clients admin to login in to my desktop to set it up again, which is a real pain.

Everything else is just copying the files over and since the new user has admin rights you just need to run sudo with the cp commands

Me too, My iMac 27 inches retina 5k 2019 , Everything was slow down after update Big Sur. Please fix this problem.

THIS!

Big Sur has ruined my MacBook, it's unbelievable! My daughter's 6 year old Dell works faster right now!

Me too with a 2014 Retina iMac.
There were some quirks with Big Sur Release but 11.1 upgrade ruined the fun completely.
A separate account seems to help, but I am really afraid of moving all my stuff to one. I develop xplatform and created an installer setup with a Parallels Windos vm for my Mac compiled app – and I don’t, don’t, don’t want to set all the pathes manually again.

Disabling Dropbox and CreativeCloud brought some relief, but once I build apps things are starting to slow down again tremendously. But at least no kernel panics and no enforced user account relogins anymore for a whole day now.

I noticed that my Mac has a tendency to use only half of its memory. After one day of uptime, I still have almost 14 Gigs of 32 marked as free, but the system swap file is 3 GB in size,using half of it actively.
Shouldn’t it only start to page that heavily when RAM is really used up?

@UBogun That's what's frustrating about this. It's not CPU processing or memory consumption that's causing this slow down. If it would be I would be able to pin it down to a certain application or activity that's causing it. Like you, I still have most of the RAM free and CPU is not really working hard either. I get the feeling it has something to do with accessing the hard drive but I might be wrong

I also have now experienced this problem. My Mac has slowed to a point of my first Dell in 1990 on phone line. Even used Clean my Mac out of desperation. It helped somewhat, at least i was able to reboot in safe mode. This sucks. It is heartening to know I'm not alone and the problem is WIDESPREAD!!!! Damn you Apple...do something. my computer is worthless. I can now eat lunch in-between sites.

For iMac users seeing a slow down with Big Sur that are *not* running Avast:

1. Does your iMac have a fusion drive? (large spinning disk, small SSD cache 'in front' of it)

2. Did you upgrade from Catalina, from an older OS like Mojave, or did you perform a 'clean-install' of Big Sur by booting from a flash drive installer and reformatting the hard drive?

3. Is it possible that the fusion drive configuration has been messed up by the upgrade / reformat process?

If so, then your iMac could be effectively running directly from the hard drive alone - without the benefit of the SSD cache - and the fix would be to rebuild the fusion drive and reinstall from scratch.

After trying everything, the solution to the problem was to clean install and don't mess with Time Machine.

Big Sur messes up with the profile. So if you clean install and restore the profile, you restore also the slowness together with it. The best way is to just copy-paste all your files from a backup and install the apps from scratch. I did this and now my 27" 2017 1TB fusion iMac boots in 30 seconds!

@jonathon-b  this means I have to wipe the mac clean and start again? How is this any better than windows? The thing that Mac has over windows is that it never crashes and doesn't slow down from year to year. Since I "upgraded" It slowed down to a crawl and it crashed twice.

First of all updating to Big sur on 128GB macbook was already a pain, then after update inspite of getting better its crying for me. It takes 3X more time than earlier to boot, plus all the apps become slower to launch. Also working with multiple app crashes the mac. Did't like the new spotlight, earlier after search quick preview was there on right without pressing enter.
Overall not a happy update.

Big Sur kill my machine: Mac (Retina 5K, 27-inch, Late 2015) 1TB Fusion Drive is very slow safari takes too much time and the startup time is a nightmare takes 3-4 minute Xcode lagging :(

Big Sur has also ruined my MacBook. Computer is now very slow and I highly regret the upgrade. Would be nice if Apple addressed this as it is a trending problem that should not be ignored.

MacBook Pro (15-inch, 2017)

Same thing happened with me I have iMac 5K 2019, everything works fine until Bigsur
Now my iMac take horrible time to run anything, struggling on boot
(my big mistake buying apple products)