Thank you for the online example.
Loading the ExtJS script took almost 11 seconds.
I see it is 2.8 megabytes. It is not a release version "ext-all.js". It is a debug version "ext-all-debug.js".
Please set up ScriptMode="Release" for a ResourceManager or scriptMode="Release" in the extnet section within a Web.config.
Another issue is the fact that this file and other resources are requested with WebResource.axd. I think that it doesn't GZip by default.
I recommend to enable the Ext.NET resource handler. There should be the following settings in a Web.config.
<add path="*/ext.axd" verb="*" type="Ext.Net.ResourceHandler" validate="false"/>
The system.webServer section is required for running ASP.NET AJAX under Internet Information Services 7.0.
It is not necessary for previous version of IIS.
<add name="DirectRequestHandler" verb="*" path="*/ext.axd" preCondition="integratedMode" type="Ext.Net.ResourceHandler"/>
If you apply these suggestions, the main resource will weigh something about 400KB. It will reduce loading time much.
And, as I can see, there is no problem at rendering stage. The page is rendered within a second when the resources are loaded (tested with FireFox).
Hope this helps.