dynamic pages vs static pages
It is probably because most of this new site is being fed by a database. Every forum page probably has to access a database many times to grab all of the post information, etc. before it can be sent to the user. Before (I assume) everything was static except for the forums and similar dynamic pages - databases didn't have to be accessed for every page. Just wait up a bit and I'm sure JP will be shoring up their hardware if it is that noticeable (10 sec on dsl! - my loading speed is just fine).
BTW, JP: What database server are you using? MySQL, Oracle? Good to know you used PHP, though :)