All performers rehearse. You’d never expect an actor to perform Shakespeare after a single reading of the script. Nor would you expect a musician to get up on stage and perform flawlessly just because you know that they can read the music that’s in front of them. If you’re trying to get the best performance from an artist then you need to ensure that they’ve had ample time to rehearse.
DMs may not be artists in the same sense or category as actors or musicians (and I’m not going to insult anyone who falls into one of those two categories by suggesting otherwise) but we can look to these disciplines and borrow their best practices. DMs certainly do their fare share of prep work when they build encounters. They choose monsters, draw maps and compose skill challenges, as well as develop the entire campaign arc. But prep isn’t the same as rehearsal.
Over the past couple of months I’ve been the DM for two different groups playing D&D Encounters. Every week I run the exact same encounter twice. None of the players or characters are the same, but everything else is identical. What I’ve found is that I’m a lot more comfortable when I run that second group and I’ve come to realize that’s it because of the familiarity I gained by running it twice. The first group provides me, as the DM, with a chance to rehearse the encounter.