We have had a lot of feedback about our Drupal site running very slow on a different network. Most of the users on this network use IE (IE6 and IE8), which is causing a huge slowdown compared with Firefox.

However, even in Firefox it is still slower than what we would expect. We have put this down to a local DNS issue (the DNS request is timing out after 2 seconds, then works off another DNS sever. I am assuming this is adding a 2 second latency to all page loads?)

What other problem could be causing a dramatic slow down for Drupal, only in IE?

Site in FF from YSlow:

Empty Cache = Total Weight - 586.2K
Primed Cache = Total Weight - 267.3K