In Q3 2013 the script that creates the modal background for our dialogs (e.g., RadAlert, RadConfirm) was improved with the following:
- some issues that allowed focus to get to the background were fixed
- accessKey handling was improved
- an option was added to set the disabled attribute of elements the developer chooses
- tabIndex recognition was improved (the key problem is that if the attribute is not set, no two browsers return the same or proper information)

Since these scripts now do a lot more work, huge pages can experience a slowdown when a modal dialog is shown. For example, a page with about 7000 elements took about 800ms vs about 70ms with the old scripts for the same page.

The performance can be improved by adding the attached script (a minified version of the old code) to the end of the page. You can add it as a script reference, or simply copy it inline, depending on your preferences. Using this script will, however, remove the fixes and improvements done in Q3 2013, as well as any other modifications we make in the future.

If a performance improvement in the scripts is possible, it will speed up the process (probably not all the way to the previous levels) without losing the new functionality.

When using DOCTYPE //W3C//DTD XHTML 1.0 Strict//EN and setting height to 100px 
RadWindow AutoResize=True gives problems in IE10 by adding unnecessary scrollbars, and in Chrome, FF and Safari the window is to high (approx. 55px too much).

Refers to forum entry: http://www.telerik.com/community/forums/aspnet-ajax/window/autosize-behavior-question.aspx
(I used the same sample project to replicate the problem).

 Please see attached screenshot and project.

A workaround is to disable animations. You can do this in the control markup/code-behind or JavaScript depending on your case.

In case this is not an option, you can disable the animations prior to closing so that autosizing does not use them:
function CloseAndRebind() {
	GetRadWindow().BrowserWindow.refreshGrid();
	setTimeout(function () {
		var wnd = GetRadWindow();
		wnd.set_animation(0);//disable animations
		wnd.close();
	}, 0);
}

You can find the detailed discussion here http://www.telerik.com/forums/issue-with-radwindow-(2014-2-618-45)