P-51 accelerates slowly, so this is probably why it seems like a sloth. For such a heavy plane, the 1650hp merlin engine is a bit lacking, but the radiative exhaust effect helps provide thrust (approximately 1000 pounds of thrust from the engine, 300 pounds of thrust from the exhaust).
So what this means is that you always want to put yourself in a position where you can dive on your opponent. In dire circumstances, you can dogfight in the P51, but be sure to use the 'vertical'.