I also noticed the same problem compared to previous versions, the amount of lag and performance degradation is insane. Graphics rendering is so slow and the longer you use brave the slower performance it gets. I refreshed and made a new profile which helped but after a few hours of use the laggy performance came back. I have all drivers updated and even tried turning off Hardware Acceleration and no luck.
I went to check out Firefox 93 to see how everything compares and what a huge difference compared to the latest version of Brave, no lag and you can open tons of sites at a time with heavy graphics with no lag, just smooth performance. I tried everything I can think of from making new profiles, reinstalling and no luck.
I also find that after about one hour of heavy usage the performance goes downhill very badly of the entire browser like the browser is failing to clean up memory and do trash collection, I have 64 GB of memory but turning off the browser for a few minutes and turning it back on makes a big improvement.