Some editors (incl myself) are experiencing long delays when typing into the WikiEditor edit window, in some pages, on various wikis.
I.e. We type a string of characters, and it takes a full second or more before the text appears.
We can reproduce in Firefox 48 and Chromium 51 (but not in Chromium 37) on Linux, Chromium 52 on Windows 10, and Firefox 47 in OSX, but not in Chrome 54 in OSX.
Logged-out as well as logged-in.
Test case 1: https://en.wikipedia.org/w/index.php?title=Thomas_Jefferson&action=edit
Test case 2: https://meta.wikimedia.org/w/index.php?title=Mailing_lists/Overview&action=edit
Taking the text from that article, and pasting it into certain pages at metawiki or mediawiki, has the same result, but not consistently.
There's slightly less of a delay at [[w:en:Barack_Obama]] (100k bigger), so it's probably not directly related to text length.
Someone's testing indicated a correlation with page-protection, but I can't confirm that.
Various reports say it has been a problem for a few weeks.
@ori noted in IRC regarding Firefox:
it looks like there is a paint / layout event on every keystroke