One thing I notice: too many things cause unresponsiveness. 8 cores in my laptop and I can't install a package in the background, and semantic parsing interrupts typing. There doesn't seem to be any concurrency at all, things I do every day like eval'ing forms to REPL cause stutters.
I love that it allows makers to explore new ui paradigms (magit, lispy), but performance is its biggest downside.
Personally I don't see performance as a major issue, though some things can be a bit annoying. But note that the concurrency problem has been recognized by the devs and a recent version of Emacs introduced some kind of threads. So I think we can expext progress in the next coming years.
That looks like an interactive ui--I can't declare use-package forms in my init.el, and have the initial install and any updates be async and concurrent.
I love that it allows makers to explore new ui paradigms (magit, lispy), but performance is its biggest downside.