I occasionally notice slow downs as well (lag). I chalk it up to server load though.

I usually have the main forum page open first, then open anything else in a new tab in browser. When I notice extended times on page loads I go back to the first page and refresh it. When the page finally reloads, scroll down next to the bottom and note how many users are on the forum. When I've notice page load lag, there was always 1200 or more users on line.