Please note that the RadSlider control is functioning properly under IE8 or under Internet Explorer browser in IE8 mode. There are known slowdown issues with Internet Explorer 8 and the behavior that you have encountered is a result of the default performance of the browser, over which we have no control.
so that we can inspect it and determine what is causing it.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed