ISA Proxy and Slow Web Pages...

If you have an ISA2004/2006 Proxy server setup and the pages appear to be sluggish this is because your MTU discovery is not enabled on the relevent server(s), to enable to feature which is disabled by default you will need to edit the follow registry key..

HKLMCurrentControlSetServicesTCPIPParameters

When you have navigated to this branch in the registry you are looking for the value named:

EnablePMTUDiscovery=1

Why does this work?
"Enabling the setting causes TCP to attempt to discover the Maximum Transmission Unit (MTU or largest packet size) over the path to a remote host. By discovering the Path MTU and limiting TCP segments to this size, TCP can eliminate fragmentation at routers along the path that connect networks with different MTUs"