never heard that one before...i thought i'd heard it all, guess it's a matter of what one would call "high altitude". that means sources are wrong...
maximum speeds (not necessarily operational speeds)
yak-3 447mph at 18,000ft
yak-9d 374mph at 10,000ft
yak-9u 434mph at 16,400ft
la5-fn 402mph at 16,000ft
la-7 423mph at 19,000ft
mig-3 398mph at 22,000ft
fw-190a8 405mph at 19.400ft
fw-190d9 426mph at 21,600ft
Speaking in a relative sense of course. German aircraft tended to perform at very high altitude than the Soviet's stuff did, and so the Soviets didn't frequent where they didn't fight real well. This isn't to say they didn't go up that high, just to say that they stayed lower for the most part. Look at the La-7, its performance advantage is greatest right there on the deck.
Also, look at the evolution of the 109's speed charts. The E4 peaks at around 15k, the F-4 at 19k, the G-2 at about 22k. And from there on, it levels out. The G-6, and K4 both have similar optimal alttudes, while the G-14 is just a bit of an oddball.
Notice the 190's also peak out at around 20K, as do the 110G, and Me 410. The non high-alt spitfires peak at around 20K, along with the Typhoon.
Not entirely sure why, but the trend of increasing optimal altitudes seems to plateau at around 20K, despite being able to tune aircraft for higher altitude, as evidenced by the P-51's, P-47, P-38, Spitfire Mk IX and XIV, Ta-152, and Mig-3 (a high-altitude fighter). If I had to guess, it would probably be the result of increased expense, and lack of return benefit for non-escort fighters (that is to say, combat altitude kind of plateaued as well, save for escorts and interceptors).
And IIRC, the Germans had a directive to avoid combat with Soviet aircraft at low altitude, due to the Soviet fighters being superior lower down.
