Hi! Videos across YouTube, Twitch and Netflix were all lagging for me after the Big Sur update on my 2017 13" Air, this is how I fixed it!
Disabled VP9 decoder and VP9 decoder on battery Reset SMC - https://support.apple.com/en-us/HT201295 Reset NVRAM and PRAM - https://support.apple.com/en-gb/HT204063 Hope it works for you guys!
winnrose
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