Betas Are Usually Slow
Even the full release of Big Sur and iOS 14 won’t be fully optimized, as this is an ongoing process that requires lots of behind-the-scenes work. Betas aren’t optimized at all. Far from it, they are infamous for causing slowdowns for pretty much all activities. Even if you can get an app or service running, expect everything you do to take longer and include frequent pauses. This may not brick your device, but it does get annoying quickly.