The editor is a bit slow starting up normally, but after you get going it seems fine. Long posts, posts with pictures, complex images, or something similar, can take up to 30 seconds on my machine just to input a single letter. My whole browser locks up in the meantime, and sometimes - but not every time - if I type in a string of letters it will take the whole thing.
I notice as well this when trying to edit my thread for Sanity. Sometimes it will even go so far as to time out with my browser telling me a script from cursecdn.com is being unresponsive and asking if I want to terminate it.
It's really annoying when it takes 30 seconds for something I type to appear. I hope this gets fixed soon.
Obligatory Info: Waterfox 28.0 or Firefox 30.0; Windows 7 Home Premium, 64-bit.