My understanding of the F4U vs F6F speed difference was from design considerations. The F4U was designed in the late thirties with specifications requiring speed, more speed, and above all ... speed. The F6F was optimized to kill the japanese zero (which was a slow plane) and its design was an extension of the F4F Northrop Grumman design. The F4U was designed and optimized (even before WW2) to have minimum drag for the engine configuration used. I have read many reports on the XF4U-1 from the NACA Langley wind tunnels on drag reduction. The F6F was also tested, but not as thoroughly (due to the urgency of the war). Also, as noted before due to the landing gear strength concerns, the F4U wing root makes a 90 deg angle with the airframe which minimizes drag.
The F4U was raced successfully after WW2, but I don't think the F6F was ever raced. If so, I never read about it. While the F6F was a great design, and held a higer kill/death ratio than the F4U, the F4U was designed to make it a faster plane. Along with the wing root drag reduction, the F6F was designed as a lower wing loaded plane with about the same weight as the F4U. That requires the F6F to have more wing surface than the F4U which generates more drag. The F6F could turn better than the F4U because of the lower wing loading, but that lower wing loading also made it slower. That's oK if you are still faster than your opponent, which in this case was the zero.
Regards,
Malta