the reason the p40 was slow was about weight and drag (primarily drag)and the fact that while the engine has plenty of power, high altitude performance was poor. If you notice, most top speeds are at twenty thousand feet or higher, and the p40 lacked a two stage supercharger. you'll notice that the P51 was this way also until they switched to the merlin, and then WOWEE! later models of P40's had more powerful engines, but other things added weight, so performance stayed about the same throughout the series. The drag factor also stayed about the same throughout because the radiator was in the front of the airplane under the engine (many pilots liked this in ground strafing because the troops had trouble hitting it, as apposed to the mustang radiator being under the fuselage). The wings had no sweep on the front (or very little, im not for certain) unlike on a mustang or other fighter where there is a visible amount of tapering toward the rear (not much, but some), the landing gear had fairings in front, and most of all it was a razor back canopy. The whole airframe really needed a good cleaning up to add some speed, and why the curtiss engineers never did it is beyond me.
