So, having now installed the beta, I can answer some of my own keyboard questions:
- there is still a very noticeable delay when a third-party keyboard loads, especially if it has not been used recently.
- there is still no slide animation when a keyboard appears. It just pops in, sometimes with a bit of flicker.
- I have not had the keyboard fail to load with Spotlight, but it's too early to say it doesn't happen.
- Messages still does not play nice with third-party keyboards. Often just reverts to the built-in keyboard for a thread, sometimes with the entire globe icon disappearing so you can't even switch back.
- Using third-party keyboards in Safari text boxes often causes the whole Browser to freeze up, requiring you to close it from the task switcher.
So in summary, I have pretty much all the same keyboard problems from 8.1.3, plus a new problem (Safari freezing) that I've never encountered before. If you were upgrading hoping for improved keyboard stability, keep waiting.