When you have choosen your party that you want to raid with, you click a “Ready” button which also can be clicked to unready and switch party.
Raid counter still ticking down but 20 people there? No more waiting. All 20 trainers click ready and the raid begins.
The wait for raid is a long and not needed once you have already set up your party you want to use. The “Ready” button will speed things up.
Have a private party? 5 players? no need to wait for timer! If the desired number is less than 20 click the “Lets play” button which will promt the game to start without max players.