Speaking as recent(4-5 months) Emacs(spacemacs) convert - it is slower compared to even "decked out" vim. An important caveat to mention is that it very much depends on what modes you are operating in -- each file type will usually operate in its own mode, with varying performance. In my experience the lagginess is rare, but its very workflow dependent, there's this one large(1.5k line) php file I have to edit semi-regularly on which I switch off the php mode, just to avoid the pain of jankiness. It isn't very noticeable in other, "regular" sized files.

But overall I'm a very happy emacs convert. It's extremely malleable and pleasant development environment and I think it definitely deserves all the gushing articles/posts its been receiving past few years.