Look at this Fiddler capture: ScriptManager loads more than 400 MBytes of JavaScript code!
WebResource.axd: 20 KB
ScriptResource.axd: 260 KBytes
ScriptResource.axd: 65 KBytes
ScriptResource.axd: 22 KBytes
ScriptResource.axd: 9 KBytes
This will make people want to come back to plain old HTML.
