iMac slows down after Big Sur installing

Agree, I bought a brand new iMac 5k 2019 in the summer of 2020 and it was super sprightly until I installed Big Sur and it’s super slow to boot and sometimes tediously slow just to use. I created an external SSD boot drive and installed 32gb of ram and it takes 52 seconds to boot up!!! What the hell!!! I’ve got a 2009 MacBook that boots quicker Han this 6 core i5. What’s going on apple???? Very poor show.

Unfortunately I too have the same problem! I have MacBook Pro(Retina, 13-inch Early 2015) 2.7 GHz Dual-Core Intel Core i5 with 8 GB 1867 MHz DDR3 memory and Intel Iris 6100 1526 MB Graphics, not too fancy, but still a great computer until the last update to the Big Sur... reminded me how my all my iPhones slow down when they get their 2nd or 3rd update.... Apple why are you doing this to us. We will buy your products, just not as often as you want us to.

OK.

I've found three things that seem to really cause big problems with 'Big Sur'.
The first is if you have a Wacom driver installed. This seems to completely ***** the boot, Many minutes delay at boot. Answer here is to remove the driver and install 'afresh' (don't 'upgrade').
The second is certain anti-virus programs. Again remove, and install afresh. It seems that the driver they use to connect to the filesystem during boot, doesn't upgrade correctly, and has issues under Big Sur (a few like Avast, don't seem to work correctly even with the latest version).
The third is large numbers of external drives. Something seems to go wrong with the USB handling for these, with lots of USB stall messages during boot. Now what is strange here, is I added an external USB3 powered hub, and now boot with this switched off. Once the system reaches the login, turn it on. Lo and behold, boot in under 20 seconds, versus over 5 minutes!. The drives all work perfectly.
It appears that something in the USB/filesystem checking during the actual 'boot' process goes wrong when there are more than a certain number of drives attached. Attach them after boot, and it works great.
Reinstalling the Wacom driver, and switching the drives on after boot, takes the boot time down by about 15 times. The system also then seems to run properly. Things open quickly, and don't glitch. Except for a few packages that don't run completely right, the system is now usable again.

Same problem here with a 1 1/2 year old iMac 21.5 Retina 4K with a 3.6 GHz i3 and 16 GB 2400MHz DDR4. It is so slow to boot up that it takes almost 4 minutes (My Windows 10 machine with a similar amount of Ram and a similar Hard Drive takes less than 1 minute). Just opening mail takes 3 minutes and same with any other application, whether Apple or third party and I am constantly looking at that spinning colour ball. Everything is so slow that I am seriously thinking of giving up and changing everything over to my windows machine. I have owned 5 Macs for the past 20 years and this is the worst operating system I have ever encountered, including on all of my windows based machines. Steve Jobs would never have let crap like this out of the building.

I'm having the very same issue with my 2015 imac. I was low on RAM and increased it to 32gb and my CPU is operating fine. But on start up I'm finding first that it takes a few minutes and when trying to open apps I not only get the spinning wheel I also have messages that I do not have permission to open that particular app or that the app isn't responding. Sometimes it crashes altogether. This has all happened since the Big Sur update.

Exact same problem as all others with iMac & Fusion Drive. (Late 2017, 27" 5K Retina, 24GB Ram upgrade). Its running slower than the 12 yr old macbook pro it replaced....was brilliant on previous OS which is the really annoying thing.

Upgraded a couple of days ago to 11.2.2, and now machine is 90% back to Catalina boot speed!.
It is a tiny bit slower to the first part of the slider bar showing, but literally only a second from there to actually being booted.
Hurrah.
Hope other people find the same improvement.

For what it’s worth, same issue here with a 2015 iMac. Boot time is more than 5 minutes. Even after logging into the user account it still takes several minutes to get to a responsive desktop, with the dock, menu bar, background appearing one after the other, the screen going black, mouse not responding, etc. This is like a time travel experience back to Windows 95.

Once the desktop is up and the first apps are launched, the machine runs fine. Maybe a bit slower than Catalina (which can be expected with a new OS).

Appalling— slowdown still happening after last 11.3 beta update. This problem still remains months later

I am here with the exact same problem. Typing is laggy, I look at the spinning rainbow ball multiple times a day now when I have never seen it come up on this computer before. I have a 21.5” iMac mid 2017 Retina 4K, i5 with 8GB ram that has never struggled to do anything which now takes a decade to even turn on and then once it turns on, further time to even become usable. Appalling this hasn’t been fixed

Updated last week and same is happening for me (iMac 5K 2017). I got rid of my iPhone for this same reason, and hate that it's a pain to go back (if not almost impossible) to previous OS, when all I need is my devices to continue working as they were before, I mean like a day before updating. I already managed to get my hands on an NVIDIA GPU to say goodbye for good to Apple. I haven't got an exciting update in the last decade, all the oposite.

Hi, I had exactly the same problem but it seems to have been fixed (as somebody else mentioned) by uninstalling my virus protection software [Avast] and re-installing it.

Hope this helps!

I have the same problem with a iMac 27" (end of 2015).
I made a clean install of it but the problem still remains.

After Troubleshooting and updating a brand new iMac running BigSur 11.2.3. iMac would perform fine and then user would report sluggish performance specifically using adobe photoshop and indesign, during shutdown the bigsur image would hang and present with a divided color tone straight down the middle. Profile was originally migrated from an older iMac using the migration assistance. Tried creating a new profile and moving the files there which worked for a little bit, but the problem reoccured. As a final effort I reformatted, performed an SMC reset and NVRAM reset and handed it back to the user. He said it was working perfectly for the morning and then it began running slow again. The only difference was he connected to his synology NAS, as soon as he ejected it everything was working fine again. I hope this helps some of you. I'm not sure if its all externals or just network shares but that seemed to be the constant for us. At all times after set up the user either had the NAS connected or a usb external drive. Try ejecting any drives you may have or performing an SMC and NVRAM reset, (I'll note too Apple support instructed me to simply boot into safe mode, sign in and restart. I'm not sure the mechanics of this but they stated it does fix a number of odd problems for some reason).

iMac (Retina 5K, 27', 2019)
macOS Big Sur 11.2.3

Finder and disk operations very slow.
After ejecting my Synology NAS drive, everything went to normal.
It seems a problem with network drives.