In case grandparent is too elliptical for some readers --
The maximum current determines how thick the wires have to be. Since power (ability to heat or do other work in one unit of time) == current * voltage, doubling the voltage doubles the amount of power a wire of a given thickness can deliver.
Well done, I should have emphasised the principal.
Looks like the maximum power that a US kettle can produce is 110 * 20 = 2200 watts, so will take about 3000 * 2200 = 1.36 or 36% longer than an EU kettle to boil.
Microwaves have their own problems - they can superheat water without boiling it, which will then explode in a fury of scalding foam as soon as you jostle it.
They're also frequently mounted overhead, where it's more dangerous to retrieve a mug or big Pyrex cup. Contrast that with the kettle which has that little flap over the spout and is spill resistant.
