Because users are typically running with firebug enabled, and caching disabled. This could be an issue with inspection of http2 traffic having more overhead when you are listening in... not an issue with the protocol or the test.

I'm not saying the test is "fair" but what you're describing isn't a fair scenario at all. I'm not sure if the latest fiddler or other dev proxy supports http2, but that might be a better way to judge.

Also, are you sure the the version of Konqueror you are using is actually using http2, or a fallback to 1.1?