Because its way too early, while using a beta, to say what the final 9.2 release will be like. Just as you say, not ALWAYS fixed, but yes things do get fixed along the way.
Things are turned on and off, both visible features and features.running processes under the hood, in betas for obvious reasons. You cannot equate a beta performance to what is released in the official release.
Betas, even public betas are install at your own risk and always have been.
I've never seen a performance difference between a final beta and the public release.