From the sounds of it, I'd say that it's the clutch engaging. From a standstill, it takes a few revs to get the clutch properly engaged and driving away. Not enough revs, and you'll stall or bog it down. Too much, and you'll wheelspin.
Even on cars with racing transmissions (And thus, no operational clutch pedal), the car still requires some revs to get the clutch engaged from a stop. The key to getting a good launch is finding that sweet spot in the throttle range (Generally half or thereabouts, depending on things like the power of the car, grip of the tyres, etc) and modulating the throttle to keep it from bogging or wheel spinning.
Hope this helps!! 👍