Ajax control\web part 3 time lag when accessed through TMG

I have a public , branded SharePoint site with a visual web part that has an asp.net treeview control.  The performance has been fine until a few days ago when expanding a node began to take a very long time instead of being near-immediate.  When the problem first began, it was taking 2-3 minutes.  It's now down to 6 seconds which is better, but not unacceptable. The performance of page loads hasn't changed.  It's just the treeview expansion that's changed.

Here's the kicker.  When accessed from inside our firewall or from on the server itself, it works great.  It's only an issue from requests made through Threat Management Gateway (TMS).  

I'm having a difficult time troubleshooting this.  I know from using browser development tools that when the treeview is expanded, an ajax call is made to WebResource.axd with a number of parameters.  The TreeView's expansion event is running a CAML query against a document library.  Since the initial page load contains a caml quary against the same library and the page load is still fine, I have a hard time thinking it's an issue with the CAML query.  

That leaves TMG, but I don't understand why all requests through the firewall would be fine except those made by the treeview control.  We also have an asp.net treeview  control with nearly identical behavior on another site in the farm, published through the same firewall, and there are no issues there.

Any thoughts would be greatly appreciated.

Nate