tree view experiences has lag time for different users.

My treeview control has nodes that expands when clicked, with a call to WebResource.axd?.  There is a lag time in this expansion depending on the end user's location.  The tree view expansion either takes a fraction of a second, or around 3.5 seconds, which is barely unacceptable to our user base and is holding up deployment.  There is not a lot of information being displayed on the node click.
Normal page load times are fine for all people.  It's just the tree view expansion.
We never experience the lag within our firewall.  When accessed externally through our Threat Management Gateway (TMG) server, results are mixed - it works fine for some while others experience the lag when a node is clicked.  Our TMG logs show that ALL external requests take about the same time server-side regardless of whether the users experienced a lag or not.  In other words, the 3+ seconds delay isn't a performance issue on our server, or ...