Might a cast iron pan set on top of a microwave oven affect the operation? Our microwave seems to take longer to get the job done these days. I notice that someone is storing a large, heavy cast iron pan on the top of the microwave. Is there any way at all that it is possible that the iron pan is interfering with the microwave?
I could try with/without the pan but it isn't that dramatic.
 A: Assuming that you have not drilled holes into your microwave (that can be really dangerous and should never be done) the enclosure forms a Faraday cage from which the microwaves cannot escape, therefore they can also not interact with the iron pan.
However, with some imagination one could certainly come up with weird reasons why a heavy weight on the top might influence, let's say, a part of the control circuit. Therefore I suggest you replace the iron pan with a stone of similar mass to assess if it is the weight or the ferromagnetic and conductive iron that causes the effect.
A: Elaborating a bit on terri's answer... If (big, hypothetical if), some object like the cast iron pan were reducing the energy going into the food, then it should be heating up. But, a big hunk of metal in firm contact with the Faraday cage that is your oven's enclosure should at most act as part of the cage.
Metal objects will in general, respond to microwaves in either of two ways (depending on how things are arranged): (1) Eddy currents will be induced in the metal; it will absorb energy and heat up. (2) It will reflect the microwave energy.
As microwave ovens age, the power they deliver does drop for a few reasons. One of them is as follows: There is a "window" between the oven's main compartment and the emitter; this window looks opaque but is transparent to microwaves. It can accumulate contamination from spilled/spattered food which will reduce the amount of energy that can pass through, reducing the amount of energy that goes into your food.
