OK, so I think we have identified the issue. What is happening is there are a lot of posts happening really fast. When you do a quick reply, there is an exchange with the server that downloads any new posts since you last loaded the page (or fetched new posts this way). Over a period of time, you grow and grow the size of the WWW page in your browser to the point it's HUGE and bogs down your computer. That people with 50 posts per page see the issue fits this diagnosis. The fix may be to just reload the browser once you start feeling any laggyness.
I also turned off animations (the slide in effect) for inserting the new posts. That might take a load off your computers.
What kind of computer do you have? When did you buy it? If it's running windows, can you tell us what the windows experience index is for your machine? http://windows.microsoft.com/en-us/...ex#What-is-windows-experience-index=windows-8
Julius I turned off all the site addons and force you to the plain default xenforo skin. If you are still experiencing the sluggish typing, we'll at least have eliminated any of the addons or the skin from being the issue.
it's just a little over a year old. It has Windows 7, and the base was 3.9, but everything other than graphics was 5.9 No other website has this lag.