Why does ASP.NET ScriptManager partial update takes longer on Machine A than B on same LAN?

I want to find the root cause of an issue I am experiencing with my ASP.NET app.

Why will a partial update for the same action on the same web form take significantly longer time for one client than another on the same LAN?

It takes so long on one client that it exceeds the default AsyncPostBackTimeout value of 90 secs for the ScriptManager tag, o开发者_如何学Gos that there is no update on the page in the affected machine.

Slow Client Machine A : IE 8, Windows 7

Normal-Speed Client Machine B : IE 6, Win XP