It might be possible that RadProgressArea polls endlessly to the server, hence producing the issue that your describe. In order to verify that please record a log using fiddler and check whether there are constant requests to the Telerik.RadUploadProgressHandler.ashx. If so, please undertake the following steps:
1) Navigate to the web.config of your page
2) Find the appData section
3) Add the following entry
<add key="AllowCustomProgress" value="false" />
I hope that this helps.
the Telerik team
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