It's not just on Big Sur. Updated from 13.xx to 14.xx (running mojave on my dev mac). What a mistake! Super slow, can't cope fps when 2 windows are open. If sounds are playing, they are reduced to noise. Webgl runs fine but as js is no longer updating the shaders/logic on time, it's not usable anymore, especially when you must have multiple windows open to test a multiuser program. Is there an archive with older versions installers somewhere? I'd like to try a few versions back from this unusable 14.2 to pick the best till the 13.5 I have on backup.

Ok so turns out, I had both safari 13.x and the preview 13.1 installed before updating safari to 14.x I now have the preview as main browser and I can work again. What a difference!! When I run both at same time, for same tasks, I see 14.x failing royally on almost everything. Not only slow in general, fps rate not respected, js slower, html refresh on dynamic/css animated sites is ridiculous, noisy sounds etc.. Busy with a SignalR multiuser 3d game atm, I see in preview I can play smooth with 4 users in four windows open, all with debug on. All traces updated as expected just like in the previous release version I had. The 14.2 already have issues with 2 windows open, and fps goes down from steady 60 to a jumpy 30-40. Fans are going so hard my mac could take off. More disturbing, before making the preview my default browser, got 2 reports of sites that "failed" in 14.x. Like USPS.com where I was trying to track a package. No error in console, just that plain stupid and useless "A problem repeatedly occurred with [address]" with no details as on iOs ( Apple, please trace the error (if any), as some of us want to know what's wrong exactly). The same page, loads fast with zero issue in the preview 13.1, no error, no warning. Basically now with both running side by side on same machine, it's clear this browser version is so bad it should never have been released. PS: Apple, while you fix this abomination, could you also remove these insanely annoying energy warnings please? At least give us the choice to disable them for a given page.

It's not just webgl (which btw I saw also lost antialiasing), the 14.x is a disaster release. On my dev MBP I run now the 13.x preview to work because with 14.2 it's simply impossible to work with. It can't handle anything that does more than displaying static content. More disturbing, it fails to open many pages claiming there are issues (not saying what they are either). The old safari preview, Chrome etc open these with no problem, no errors or warnings. Try run rich content with both versions on same machine and be amazed how bad this version is. Really hope to see an update very soon.