Will the 410s 50mm cannon shoot relativley fast or will it be slower than the p39
Do you mean muzzle velocity or rate of fire?
The BK5 has a rate of fire of 45 rounds per minute and a muzzle velocity of 920 meters per second when firing HE rounds.
The M4 37mm gun on the P-39 has a rate of fire of 140 to 150 rounds per minute and a muzzle velocity of 610 meters per second when firing HE rounds.
As you can see, by one measure, rate of fire, the P-39's is faster and by the other measure, muzzle velocity, the Me410's BK5 is faster.