Core losses in induction motor
An induction motor's speed is controlled by the AC supply's frequency. A higher frequency means a higher speed. Given that the stator's inductance is fixed, a higher frequency reduces the magnetization currents flowing in the stator. 

This in turn reduces the onset of core saturation (caused by those magnetization currents). And, in turn, core losses decrease.

Stray inductance is caused by those stator turns that do not magnetically couple to the rotor. Stray inductance is not a power loss as such; it just reduces the power that can be fed to the rotor from the AC supply. Because it is an inductance, as operating frequency rises, the reactance also rises and this, in turn reduces the ability to push power into the rotor.

Here's the equivalent circuit of an inductor motor (single phase shown): -



The inductor marked \$jX_m\$ is the magnetization inductance I refer to. The inductor marked \$jX_1\$ is the stator's leakage inductance. It should also be noted that there is also a leakage inductor in the rotor, \$jX_2\$