The fortunes of the air arms of the various combatants in World War II were determined to a large extent by what the leaders of those combatants saw as the main "mission" of their air arms should be.
For the Russians, that mission was close air-support of the ground forces. Consequently, Russian aircraft were designed to deliver maximum performance at low-levels, where it was most needed.
The Germans preferred to carry the war to higher altitudes, at least in fighter design. Early in the war, German fighters, especially the Me-109, were designed to deliver maximum performance at altitudes of at least 20,000 feet. Because of their high-altitude performance, German fighters almost always engaged Russian aircraft from a superior tactical position. The kill claims of German fighter pilots may be exaggerated, but are probably closer to the truth than the Russians are willing to admit.
The P-51, P-38, and the P-47 displayed a high-altitude design philosophy that in many ways mirrored that of the Germans. However, the American aircraft payed certain penalties in design because they had one performance parameter that the German designs did not; long-range bomber escort. While many AH fighter jocks decry the manuverability of the Mustang it would be well to remember that in real life, low-speed manuverability wasn't really all that important. High-speed manuverability, at high-altitudes was. Long range and high-speed manuverability were the main performance goals of American designers. These designs, especially the Mustang, were imminently suited for the war that they had to fight.
By the way, I've read several articles by pilots who flew captured or restored Me-109s of various models. While their personal opinions about the merits of the 109 differ there is one thing that they tend to agree on; handling at low speeds, up to 250 mph was excellent. The 109G series could outmanuver the Mustang in a low-speed, hard-turning dogfight because its roll rate and handling were superior. Of course this was not of much use in the skies over Europe, where combat speeds were usually much faster than 250 mph.
Regards, Shuckins