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Just wondering if the lag problem is specific to Firefox, so I'm now trying out Chromium (version 28). When typing in WYSIWYG, Chromium is a lot more responsive at first (CPU around 30%). No visible lag at first, but then after a couple of minutes of typing, things start to slow down (CPU reaching around 50% sometimes).

Maybe my computer is just too old for this kind of thing? It's got a Intel Core2 Duo processor, purchased in about 2007, I think (LG R400 laptop).

Yes, I see something very similar on my lappy (atom something, 3 years old with Opera, FF seems a bit more reactive, but not that much).
It seems the WYSWIG editor is not very cheap when it comes to client-side CPU.
It happens somehow with the bare editor as well (tested at http://www.sceditor.com/ ), but not to the magnitude we are experiencing here, so part of the problem may be some of ours plugins or hacks (mine ) to integrate it with the rest of the script.