My 12 Mini has a slight performance hit - I suspect that might just be general bugginess of a x.0 release and the battery has definetly taken hit.
My iPad Pro 10.5 has also taken a hit… Noticeable lag and stutter, which is a shame.
However the iPad Pro I don’t think I’ve done a clean install on for a long time. I also ran all the iPadOS 15 betas on it so it might need a fresh install. It’s possible that it’s been through the iPadOS 13 and 14 betas as well without a clean install.
My SE 1 is definetly slower as well. Think it also needs a clean install. It seemed to be ok last week but today its been annoyingly laggy!
Unless you really want the features I’d say hold off!