We've discovered the cause of the forum index taking a while to load and it was to do with the latest topic title mod we have installed on the site. You've probably noticed the forum index looks a little different with it missing.
We think this mod adds good value to the user experience but perhaps not at the cost of the issues in speed that it's causing. The web team are going to go through this mod in more detail and discover why it's causing lag and hopefully we'll be able to fix it; if not, we'll look into implementing our own mod which displays topic titles.
So please bear with us while this function is temporarily removed and enjoy the speed!