In sci.electronics.repair, on Wed, 30 Nov 2016 15:09:58 -0800 (PST),
wrote:

When you get down to thing that only take a few seconds, the fact that it takes about two seconds for the magnetron filament to warm comes into play.

Sure. That's what I had in mind.

Usually you can hear the fan slow down once the microwaves are really being produced.

I'll pay more attention.

Thanks and thanks all.