You're right that prep time is important. Priority no 1: Dial in your grind and make lots of shots!
Setting up your work station and the judges table shouldn't take long. I think, max. 4-5 minutes total. If you've organized it well and thought it through, it's not hard to do that fast. But it requires that you have everything organized. Then spend a good amount of time on making shots. Think of it this way: the more shots you can make in that time, the better the shots will taste for the judges. Not only does it give you a chance to dial in your grind perfectly, but you also 'season' the machine and more importantly, you get more confident when you taste how awesome those shots are coming out :-)
Last, practice to clean up everything in 2 minutes, so it's looks spotless.
And have fun!