I was into the hobby back in 1981 and I remember that the chief advantage of Fast Angle wheels was as Pat Kn said: "What this does is reduce the friction between and wheel and the rail because less area is touching" the rail. In other words less "wheel" is touching the rail than with standard wheels, and so less friction is created. So, rolling stock with Fast Angle wheels roll more smoothly than PW wheels.
John Knapp
Erie, not Eerie