You mentioned you fly the 51, so that is a part of the mystery.
Some planes like the 38 and the Spit handle well at very low speeds, and it lets them do maneuvers like hammerheads, wing-overs, and such that are much more difficult in other planes. The Pony's wing particularly is designed for high-speed flight, and does not perform well in very low-speed flight regimes. So yes, if you are flying a pony, you will see other planes do low-speed maneuvers, ususally at the top of vertical moves, that you can not possibly follow.
The bottom line is this. The P51 is designed to fly fast. If you drop below about 250 Mph in the Pony you are going to be out-performed by most other planes. In the Pony speed is life. Keep the pony up around 300 Mph and very little will touch you. Drop below 200 Mph and you had better be a damn good pilot because the other planes will have the advantage.
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Lephturn - Chief Trainer
A member of The Flying Pigs
http://www.flyingpigs.com "A pig is a jolly companion, Boar, sow, barrow, or gilt --
A pig is a pal, who'll boost your morale, Though mountains may topple and tilt.
When they've blackballed, bamboozled, and burned you, When they've turned on you, Tory and Whig,
Though you may be thrown over by Tabby and Rover, You'll never go wrong with a pig, a pig,
You'll never go wrong with a pig!" -- Thomas Pynchon, "Gravity's Rainbow"