Clearly there are major issues with Big Sur running on Mac's with Intel processors. After I upgraded my 2018 MacBook Air to Big Sur I had many problems. Network issues, battery life issue, sluggish performance issues, and a lengthly boot time. I tried many suggestions from this forum and various other web sites. The way I eventually fixed it was to revert back to Catalina. I erased the hard drive, reset the NVRAM, and did a clean install of Catalina.