If you wanted highest efficientcy and speed from an AC motor what would you do? Could the rate of alternation of current be optimizes? More windings? Materials? More lamination? Other factors I haven't thought of?
You really can't change the frequency of alternating current. This is fixed at 60 hertz (cycles/sec) for USA and 50 hertz most everywhere else. by laminating the permanent magnets you reduce losses to hall effect currents on the surface. Most commercial motors have already made the most efficient design possible. You can always improve but usually only getting marginal improvements for increased costs.
Fit ceramic bearings. There are many videos on YouTube which demonstrate how much less friction these bearings provide.
If you have a large budget; Make use of active magnetic bearing technology which provides zero friction as the shaft effectively hovers within the bearing. You will also need to factor in the power usage of the active magnetic bearing controller into this, as such, this would more likely pay off in a large motor setup.
Place the motor within a strong vacuum. This will eliminate drag.
Use a permanent magnet motor which is approximately 20% more efficient than an exciter based motor.
Regarding speed: You could increase the voltage within tolerance which should increase the RPM speed. For example, if you run 220V 50hz motor, you could supply 240V or 250V to get a slight boost in speed. Note, that the motor would draw more current under load and you could damage the motor.