Yes! I'm having the same issue. Ever since upgrading to BIG SUR my fans are almost always blowing very heavily and loudly, even when I'm barely doing anything intense. I have a 2019 16 inch Macbook Pro that I purchased a couple of months ago and one of the main reasons was because I hated how hard and loud the fans would blow when I'd hop on zooms. I bought this new model and the problem did improve. But after updating to BIG SUR a few days ago the issue is back and louder than ever. But that's not even the worse part...
The worst part is I bought this laptop mainly for video editing. And a week ago before the update I was editing a project filmed in 1080p perfectly fine. Everything was as smooth as butter and fairly quiet. After updating I went back into that project yesterday and today to finish it up and no matter how many times I restart my computer and close every other app other than final cut pro, every time I start editing the footage the fans start blowing full at force and I'm experiencing a serious lag as I play the video I've been editing on my timeline. An issue I was not experiencing before the update. It's literally the same footage as last week and the only difference is I updated my OS.
I'm so upset by this, I feel like this update has ruined my smooth workflow, slowed everything down, and made the experience of editing very annoying as everything lags and the fans are super loud. I spent close to $5k to get an almost fully spec'd out laptop a couple months ago and it's now operating slower and worse than the 2015 model I had. Thanks for making that entire purchase I just a complete waste of money. I seriously hope you guys fix this issue ASAP in an update!