This side effect might be related to WebResource caching - when your website is in debug mode (<compilation debug="true" /> in web.config) files streamed by WebResource.axd are not cached by the browser.
Here is a nice blog post on that matter:
In a word everything should be ok once the site is not in debug mode (<compilation debug="false" />).
In additon, have in mind that the VS internal web server (a.k.a Cassini) does not use browser's cache.
Let me know whether this helps.
the Telerik team
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