I agree with you, Sol. Variety is important, so is being aware of the mood of your audience.

Another factor: I bet that if the impatient people timed the people who fail to sing immediately, that the impatient people would discover that the delay is a matter of a few seconds. Probably less than 30.

Where do people get the idea that their needs should be met immediately, if not before? I suppose it comes from watching TV. On TV, the announcer announces a singer, the camera goes to the singer's face, and the singer starts immediately. We get used to that. Never mind that it's all preprogrammed, rehearsed and videotaped. And the singer isn't gazing earnestly into our eyes, but into a teleprompter.

(I guess that there's very little of that kind of programming any more. Nonetheless, we're used to it.)
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I used to get very irritated when babies cried in church until a priest pointed out to me that when they cry, they cry for only a few seconds. I'd never even thought of that.