I checked out that log that you linked to macmanx. I thought I might provide some more useful information.
I too noticed a lag when using the current version of Firefox (5.0). All the things that were previously mentioned have been tried (properly) and still to no avail. I, like the previous users, am running Windows 7 (I don't know about them, but I have the Home Premium x64 version). I wouldn't totally write it off as a bug to TinMCE though without some further investigation. You see, for some reason, many people who now have Windows 7 tend to have sporadic "DNS server connection" issues, issues that nobody (even super tech guys) can figure out why it's happening or how to fix it. Personally, I would think that if it was a problem with TinyMCE, that a broader range of users, if not all, would experience these problems, though if you guys detected significant lag on a beastly machine, then maybe whatever is the problem with Windows 7 is exacerbating the problem with TinyMCE?
I don't know though, I'm smarter than the average Joe when it comes to computers/hardware/software/the Internet, but I don't really know enough to say any of that for certain, I'm just working on normal real-world logic (which I know often does not apply to computers, as I learned via LSL scripting -_-*)
Now, backspacing and scrolling are not the only issues for me. I do have those, but there are others:
When I click on a post to edit it (from the WordPress.com dashboard), it loads extremely slow.
Clicking Update or Post causes extreme lag and really long loading times.
I also clicked the "Add Audio" button one a post and it took it forever to load the "pop-up." I closed it, but clicking "Add Audio" or "Add Video" or any of those set of buttons, it loads instantly. However, even if I've been on the same post, I click update or save, and it loads forever, even if I've not navigated away from the page.
I don't know if any of that helps, but at least you guys have gotten to the root of the problem (hopefully?).