Bootcamp performance drops after installing Bigsur (Irreversible)

Hello guys, does anybody got slowed down Bootcamp performance after updating from Catalina -> Big Sur? Not only on gaming, but overall Windows 10 performance! For example, it takes longer to open/install programs etc. After googling around, I realized that big sur has a problem with Bootcamp, so I downgraded my OS back to Catalina by using a bootable installer on my external hard disk that was downloaded from the official Windows website. After the process, all of my mac's data was erased so it started from 0 again like new.

After successfully re-installing Catalina OS again, I installed windows 10 on bootcamp, and install the AMD driver from bootcampdriver.com too. But the performance issue didn't change that much, bottom line is my mac didn't run as smooth as before I installed Big Sur, even after I uninstalled it!

I tried to find a solution on google. Some said it's a problem of a bad windows 10 update ( KB5001330 and KB5001337 ) so I remove the update, and others said there's a malware/virus, so I downloaded malware bytes to remove those. Tried both solutions, but the performance still isn't getting better. (especially low fps on gaming)

Is my bootcamp permanently damaged or something because of Big Sur? I'm sure that disk space won't be an issue, since all of my data was erased after reinstalling the OS. So it doesn't make sense now that my mac has full free space, but it runs slower than before.

Please kindly help me solve this issue, I miss playing games and using windows on mac with good performance.

Thank you before!

PS: I'm using 2019 iMac, 27inch, 8gb ram, 2tb hdd, 5k. intel-core i5.