After typing in one sentence, I can watch the letters appear one by one over the course of half a minute or so.
Topic on 2017 wikitext editor/Feedback
@Chermago, thanks for your comment. That's not how it's supposed to work, of course. Mind sharing an example? Browser, operating system, skin? Getting the console log would likely help terribly, here.
Well, it appears to be related to JavaScript execution in general being a little slow over here (old machine, two big browsers, quite an amount of open tabs), and this new editor being much more JS-heavy. Because, frankly, also the traditional editor is quite slow, but not nearly as slow. From time to time some random script goes rampage and keeps one CPU thread at 100 % load. Then it's really devastating if responsiveness depends on executing some (competing) JavaScript.
For me, they appear about 1/10th of a second after pressing a button and that is already very annoying because one is used to them being visible instantly.
@Chermago, is there any chance that you're using Safari 11? I've been having problems with it. The Activity Monitor shows that the memory is slammed every day or two.
Oh, and would you mind trying something else? Try editing a page with &safemode=1 added to the end of the URL. If you have your prefs set to two tabs (not everyone does), then https://de.wikipedia.org/wiki/Plötzlicher_Kindstod?veaction=editsource&safemode=1 should work. This will put your account in safemode, and if things are quite a bit better that way, then we have a better idea of where to look for the problem.