It might be just initializing stuff and texture upload to the card? It's common knowledge that starting any complex 3D game can cause slowdowns until the card has all the stuff it needs... in-game texture etc swapping isn't so massive so it does not cause slowdowns. There are also techniques called "warming up the card" like rotating a camera in game world 360 before showing anyting on screen. But your code is pretty simple so it might not be the case here.