Just updated to Big Sur few days ago and have the same issue. Restarting with cmd+opt+P+R helped but not for long. It lasted a day and both browsers, Safari and Chrome as very slow. Did anyone solve this issue yet? Hope Apple is looking into it.

Any answers yet? I just upgraded to the Big Sur and my screen is being taken over by the flickering lines. I can't work on the computer like that. MacBook Pro (13-inch, 2019, Four Thunderbolt 3 ports)