I did some real quick on the deck speed test for the two P-39s
P-39D 308MPH
P-39Q (no wing guns) 308, 329 (WEP)
P-39Q (w/wing guns) 301, 322 (WEP)
For comparison
P-40E 282, 297
Spit V 289, 304
Ki61 303, 313
109E 283, 293
The D looks to be pretty fast for an early war ride, but the Q is not great considering its competition in mid-war.