If a large AC generator is overloaded, will it lose frequency or voltage?
It depends on whether the weak link is the generator itself or the prime mover.

If the construction of the generator limits its power output &mdash; the resistance of the wire, the strength of the magnetic field, etc. &mdash; then the voltage will sag, even while the torque on the shaft remains within the capability of the prime mover, so the frequency will not change appreciably.

On the other hand, if the prime mover is undersized with respect to the torque that the generator can produce, then it will slow down, and both the frequency and the voltage will drop.