I'm posting the reply to your support ticket for public reference.
I can confirm this issue. The reason seems to be that we've stopped setting the Last-Modified header as part of a previous fix. This means that the browser can't send If-Modified-Since and "soft" refreshes will get 200 OK instead of 304 Not Modified.
The good news is that the resources will be normally cached - only explicit Refresh commands will re-request them.
We'll fix this right away. The fix will be available in the next internal build and in the upcoming service pack.
All the best,
the Telerik team
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