Speed can help you escape combat, or initiate it, but once combat has started, speed isn't as much of an asset.
Once combat has started, the more important factors are manuverability, acceleration, and climb.
The spitfire is easier to fly in combat, because it requires less pilot skill to be sucessfull in combat, because it doesn't require you to know how to use climb, acceleration, roll, or anything other than turning to beat an opponent (as long as you aren't dumb and don't try turning with zekes or anything).
1) I don't like hordes, boring after a while, and they're the only source of a longer engagment
2) I don't like ho'ing, says virtually nothing about whos the better pilot
3) I don't like turning birds that when a less skilled pilot see you can out climb them, out roll them, or out fly them, they imediatly start the low down in the weeds turning defense
combine all 3 of those into one plane, and you get the spitfire. Easy to see why I don't like them, Sam I Am.