Same here.
Just reinstalled OS X Catalina (that's the version before Big Sur). Finally got my laptop back in life! Having Big Sur for more than half a year was really painful, playing a video was laggy, and the laptop was hitting high temperatures even with fan spinning on max. It was also significantly slower with all other programs.
Strongly suggest to everyone to get some time to do this:
https://discussions.apple.com/thread/252046511
https://apps.apple.com/us/app/macos-catalina/id1466841314?mt=12
https://support.apple.com/en-gb/HT201372
MacBook Pro (Retina, 13-inch, Early 2015)
2,7 GHz Dual-Core Intel Core i5
8 GB 1867 MHz DDR3
Intel Iris Graphics 6100 1536 MB
znjrols
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