The P-40 more than other planes needs flaps working at various speeds. The first notch of flaps comes out at high speed and helps high speed breaking turns as well as when you really have to dump some speed to get a kill (before the target turns the tables and shoots you down).
Historically they had very quick (from 0 to full deflection in 1-2 seconds) flaps operated by a hydraulic system. You just pulled the lever and the flaps moved until you stopped, meaning you could choose any setting of degrees you wish.
In-game this means that even if you only had 1 notch, 10 degrees or whatever it is, the first hit would "jam" that flap and you're toast. Worse yet, the second hit often "jammed" the other flap. It's some odd quirk of the damage model where the hitbox for the flaps occupies the entire airframe so any round hits it first.

But the end result is it really gets you killed fast. It would be nice to see if fixed, same as "first hit kills a gun" issue on F6F before it was remodeled.