I don't know if microwave ovens still use ferroresonant transformer
supplies. I'd heard that they'd moved to switchers but have not
worked on any with same. It would make sense: good transformer iron
& copper ain't cheap... and a switcher would also save shipping weight.

And yes, the clock would run fast.



Some use switchers, they'll be much lighter and say "Inverter"
prominently on the front of the unit.

I've never seen one with a ferro-resonant transformer. They use a
standard laminated core power transformer with a pair of magnetic shunts
to regulate the current. Every one that I've ever worked on has exactly
the same circuit, 2KV RMS transformer feeds a charge pump doubler
consisting of a diode and capacitor, feeding 4KV pulsed DC to the
magnetron. Heater voltage is obtained from a tap at one end of the
secondary, usually physically it's several turns of much heavier wire
next to the HV winding wired in series with it.