Frustated after Big Sur Update - Wifi Issue

In catlina wifi was working perfectly fine. No slowness, connect automatically without any issue. After Big Sur update wifi take time to connect after disconnect. Speed is very low as compare to other laptop. Tried all ways mentioned in apple community discussions. I am a developer, due to this issue I am loosing productivity. Need a solid fix for this.

Firefox is quite faster then safari. Don't understand why speed is blocked while browising through safari.

I want to go back to Catalina MACOS. But not able to download installer from App Store or any other apple provided links.

Please help me to get out of this.