Why is there a delay before ATC responds?

In short, because the amount of processing going on behind the scenes is significant. We are, of course, keenly aware of this and working to get this shorter in any ways we can. We also fully expect that this tech will naturally become faster in the near future.

That said, a delay when communicating with ATC is not only pretty common -- it's extremely realistic. VFR pilots often take a back-seat for controller workload, and delays of even up to a minute while speaking to a real air traffic controller aren't unheard of.