This is related to the reply box, but not to the lag. When previewing a post, and the going back to the Editor option, sometimes only half the box is down, and there is no taskbar available to scroll through the text you are trying to post i you need to edit it.
typing just now and theres still lag. Though now it pauses as if firefox is going to crash then displays several words at once, instead of one character at a time. It's definately improved but still very off-putting.