Update: Problem solved for my MBP 16" by doing this:
leave beta program with the computer, if i's in
create a TimeMachine backup if needed
Reset NVRAM
Delete SSD
Reinstall the original MacOS, which came with the Mac by booting with shift-option-command-R (important! Just reinstalling the latest Catalina or Big Sur didn't work for me!)
Update to BigSur release version - no beta!
Migrate backup if needed with migration assistant
that's it, hope it helps you too!
edit: I tried to include the links to the support pages from Apple for the steps, but the Forum won't let me. So please search for the according guides in Apple Support to make sure you do the steps right.
Sherman13
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Same for me on an MBP 16" 2019 model.
Its laggy even to type in Pages. Mac OS Big Sur 11.0.1.
It seems to be related to the GPU, because turning off the automatic graphics card switching seems to improve the situation at least temporary and superficial.
It remains unclear, if it is related to a problem with Intel GPU or GPU in general and the reason it doesn't appear on the AMD GPU (5500 M, 4GB) may just be related to its increased computing power.
Hopes are up for a quick fix, I don't like the idea of going back to Catalina, but as it is right now, it destroys all the fluidity in the workflows I love so much while using my Mac.
Best regards