iMac slows down after Big Sur installing

Same here :( first load takes around 1 min, with catalina it took about 10 seconds.

Perform a clean installation iMac (Retina 5K, 27-inch, 2017) Big Sur working perfectly.
Radeon Pro 570 4 GB

Same problem with 2017 iMac. Just upgraded to Big Sur. Now launching apps takes minutes as the icons bounce up and down. No indication in Activity Monitor of anything taxing the system. Tried to return to Catalina with no success. Used Internet Recovery Mode to attempt a clean install from scratch and I'm still having the same problem. Lots of spinning rainbow wheel and waiting, waiting, waiting.

To the user saying the machine where they did a clean install is OK, after my initial problems, I did this. Reformatted , and did a clean install. The problems remained.
It seems to be some particular hardware combinations that are having problems. Possibly Apple did not get the proper beta testing done with Covid.
On a 2017 i7 Fusion machine, Big Sur for me runs fine, On my 2019 iMac Pro using an SSD, it does not. On the former it does not seem much different from Catalina. Boot is possibly one second slower, but programs run OK and really everything is fine. On the latter, I used to get faster booting 30+ years ago, loading using a tape!....

Ridiculous! 2017 iMac with fusion disk and 40Gb RAM boots slower than my 2012 Macbook pro with Catalina.The overall speeds looks fine, but the start... oh my.
Anyone have good resources on how to perform clean install and roll back from Big Sur -> Catalina?

I downloaded big sur on my 2018 mid spec MacBook Pro with touchbar and everything works Perfectly fine. Now that I’m back home I also wanted to update my 2017 4K 21.5 iMac. But it’s the same as all of you others say, loading takes forever and even something like starting Safari takes five seconds. I hope it really gets better over time.

My Imac has slowed down also. Not only in initial loading but my large files freeze photoshop and makes working with them impossible. This change was not immediate, however. Big Sur was installed a few days ago but I started noticing problems this morning. Perhaps there was an automatic update overnight. I have 48 gig of memory and my hard drives have plenty of space.

The same problem here!! 
After the update to Big Sur my Mac gone So much slower, it takes more than 2 minutes at the start-up vs Catalina 24sec!!!!
iMac (Retina 5K, 27-inch, 2017)
Processor 3.4 GHz Quad-Core Intel Core i5
memory 40 GB 2400 MHz DDR4
Graphics Radeon Pro 570 4 GB

Please fix this issue.... I feel like I'm working on Windows ;(

Same here :( windows style

Same here.

iMac Retina 4K, 21.5, 2017

• 3.4 GHz Quad-Core Intel Core i5

• 8GB 2400 MHz DDR4

• Radeon Pro 560 4 GB

Resetting SMC and NVRAM helped! iMac started to boot as usual (about 20 sec) and all programs appear ultra fast. Hope this helps you

Forgot to say: to reset SMC on iMac just pull out the power cable and wait a little bit.

It is embarrassing and frustrating to see a copy of a 3.7GB file to a flash drive in 43 minutes. My MacbookPro14,1 and an iMac19,2 are having the same problem. It's hopeless.

Yeah, it's an absolute disaster. My boot up time takes almost 2 minutes just to get to the screen to type in password to enter the OS and then after that takes another 2 minutes to the point I can start using computer. I noticed dropbox takes forever to start up where on my other computer it boots up just fine.
The boot up time is definitely frustrating. I just couldn't believe it and overall it's just sluggish and takes 10 seconds to open a browser or another program and I have a 32gb ram, i7 imac with top video card.
I reset SV and VRram and it didn't solve anything.
I am very disappointed to say the least. It's embarrassing when windows boots up faster. I also only have one login item and that's dropbox only. I have hardly any extra stuff on my computer.
The boot up times from previous version went up 300% to 500%. It's just crazy.

I am having a same problem, it looks I am working on a windows.