The forum update brought tons and tons of unnecessary javascript to the forum, which may cause lag like that. Make sure you're using a browser with good JS performance to avoid this (Chrome's the fastest at the moment, but anything other than IE is good).
Chrome having the fastest JS engine is disputed by the latest Opera builds, Opera also does better at loading Javascripts properly
http://blogs.zdnet.com/hardware/?p=7247&tag=trunk;content
http://techcrunch.com/2010/03/19/javascript-conformance-test/
Edit: to clarify, I meant disputed in the most literal sense. One pulls ahead of the other depending on build and test, no clear victor.