There are 2 loading screens that say "push space bar to continue". Well, the game starts once you see that first message. The game starts once that happens. For sure it happens during that second loading screen (which sucks by the way). It's an overhead view of the start of the mission. You can see everyone move. Using the helicopter showcase as an example, If you push space bar immediately when those screens pop up, then you start quickly with the rest of mission. If you wait for the screens to finish, the helicopters are already gone and about 30-40 seconds have gone by.