The typing goes slow inside contenteditable only with Firefox on Mac OS. In this case doesn't happen with another browsers. I try reproduce this case with Firefox on Windows OS. It works!!
I found this problem on : https://bugzilla.mozilla.org/show_bug.cgi?id=458051
And you can test on this linl : https://bug458051.bugzilla.mozilla.org/attachment.cgi?id=341294
But it haven't had any solution to solve it yet.
Pre-beta build of Firefox could solve this problem. http://www.mozilla.org/en-US/firefox/aurora/