micky explained on 11/30/2016 :
How long does it take a microwave oven to warm up?

The energy transfer is nearly immediate. The oven only warms up because
of the food warming up.

Some items which I microwave would end up having cold or cool spots, so
I used to cook say a three minute item by cooking for two minutes and
then allowing the heat to spread for a couple of minutes, and then zap
for another minute or so.

I now have a new microwave oven with a turntable (the other had a
reflector) to help avoid the spottiness. The ovens power output is also
an important factor to consider.