Running very slow after upgrade to 12.6

Update my case :

1. Restart iMac and ran in safe mode. Running was very very very slow in safe mode, it took 5s to respond for a single mouse click.
2. Re-install macOS (Monterey 12.6) by using Command + R option. After the re-installation, the iMac is running normally and smoothly.

Problem solved.

Hope this experience is helpful for other people who have similar issue.

After some google research, I just follow the steps (below) to reset SMC of my iMac, and it Solved the problem, system runs smoothly now.

• Turn off your Mac and remove the power cord from the back of the device
• Wait for at least 15 to 20 seconds and plug the power cable back in
• Finally, wait for another 5 seconds and power on your Mac as you usually would.

Great !!!!

The problem is still there, but more likely related to Safari. The system ran OK until I saw a one-hour video in youtube and another 30s video in netflix via Safari. It ran slow again (even during the video playing, video itself was smooth). And when I tried to restart the system, alerts kept popuping up saying Safari was blocking the restart. I had to force safari stop from the system menu, then restart.

After restart, the system ran obvious slowly then before but still acceptable. There was no big process running background (via the activity monitor).

Will do more tests with safari.

I re-installed the iMac with "Command+R" and did the erase HD first. The installation was stuck in the "less than a minute......" for hours. I had to follow some YouTube solutions to solve this (by using the FirstAid in Disk utility).

And, as a brand new iMac with Monterey 12.6, it ran slowly, just like out of memory. It took ~ 3 seconds to response when I click the blue tooth icon on the menu bar. Not installed any other apps, event not set up email account.

Conclusion : Monterey 12.6 is NOT good for iMac 27'' 2020. At lease not for mine.

I need to find a way to downgrade or restore to Big Sur using my time machine backups.

Latest update :

System runs smoothly after the complete re-install. But, every time see a video (I tried YouTube via safari, Netflix via safari, and directly Apple TV), the system will become slow (the video playing itself is smooth) after the video. Have to reset SMC again to bring back the performance.

Looks like in Monterey 12.6, something about video playing is not good for my iMac 2020.

Latest :

It looks like 12.6 need sometime to "warm up". It runs more smoothly after 15 mins.

Will stay for a while and see what happen.