Having worked Area Control for over 25 years I will make the comment that I have noticed an increase in the response time for control instructions. Unless my memory is getting really bad I seem to remember getting pretty prompt responses from air carriers years ago, I will admit the general avaition segment was some times slower. But the last few years it seems there is a long lag (up to 10 seconds) before i get a response for a control instruction. It is sometimes hard to wait for these delayed responses when other control actions are pending at a busy sector. You learn to wait because repeating the transmission steps on the the delayed reply and then things get really messy. I would be inerested into a pilots takes on why responses are delayed. I have discussed this with other senior controllers and they all sense the same situation.