What affects speed-up of sirens?

[Franz?]

Most likely, bearings.
2nd most likely, high degree of slip in the motor because the rotor was machined slightly undersized on Friday afternoon.

Loss of voltage to the motor on starting generally isn't a problem unless you're dropping 20% from nameplate.

Single? -v- 3? isn't a factor since the little electrons race around the stator at the same speed creating more or less the same flux field. Single ? may start slower for the first second due to flux field differences created in a single ? stator -v- a 3? stator. Generally the slower start won't be perceptable.