I have a WinForms "host" application, that contains a WebBrowser control: https://msdn.microsoft.com/en-us/library/w290k23d(v=vs.110).aspx
In this WebBrowser control I run a Web application built using "Telerik UI for ASP.NET AJAX". This works fine most of the time.
Problem occurs, when the Web application opens a RadWindow, and this RadWindow displays a WebForm from my application. Most of the time it takes 3-6 seconds from the user clicks the button to open the window until the window is displayed - and sometimes it takes addtitionally 3-5 seconds, until page inside RadWindow is loaded. The WebForm inside the RadWindow is very simple - it normally takes a fraction of a second to open.
No problem when the WebForm is opened in a popup window (using window.open) instead of inside RadWindow.
No problem when using WebDev (the development web server that comes with Visual Studio) - problem "only" occurs when the web application runs under IIS.
I turned on Fiddler to see what happened. When I start "Capture" in Fiddler, my problem goes away (when running against local IIS - running Fiddler did not make any difference when running against an IIS on a remote machine). Maybe my problem is related to the discussion in: http://stackoverflow.com/questions/10428113/why-http-request-with-fiddler-is-blazing-fast ?
My web application is built using Telerik.Web.UI.dll Version: 2014.2.724.45. I have a small demo web application, that illustrates this problem (and I have a simple test "host" application containing a WebBrowser control).
I made a copy of the web application, and updated to Telerik.Web.UI.dll Version: 2017.1.228.45 . This did not make any difference either.
If needed, I can supply the applications illustrating the problem.