I've not seen it on any other threads, just that one, and it only recently started happening. I've confirmed it with IE, Safari, and Firefox, though with the latter two it only lasts for a second or two, so it may not be noticeable. I suspect IE may be thrashing in a script somewhere, similar to what happens with long Microsoft forum threads. That's what made me mention the possibility it has to do with the length of the text.
IE's network monitor shows the last thing requested is /board/favicon.ico. Then there's a 15 second delay, then the page will then respond to scrolling. None of the requests stretches out to the end time; this implies it's something that's running locally in the browser causing the delay.