[quote=chaosmagnet]Yes, the lower voltage means they take twice as long to boil water as UK ones. We in the UK also have gas cooktops, but an electric kettle is quicker for boiling water. (Seriously: I sometimes boil water in a kettle before pouring it into a pan to use on the gas hob.)
It's the wattage, not the voltage, that matters. Apparently, years ago, high-wattage kettles were not available in the US. That's no longer the case.
Funnily enough, I'm visiting my mother and her electric tea kettle has failed and we're heating water for tea in a saucepan.
The rate at which water can be boiled in an electric kettle is indeed related to the wattage, but in the real world in most situations, the available wattage is determined by the voltage.
Here in the UK we use 240 volts and most domestic outlets are 13 amps, the highest wattage kettle is therefore about 3,100 watts.
In the USA 120 volts is used and most outlets are 15 amps, that limits kettles to about 1,800 watts or not much more than the half the UK ones with a consequent increase in the time to boil.
In theory a 120 volt, 30 amp kettle could be manufactured and would boil slightly quicker than a UK one, in practice though no one would buy it on account of the expense of instaling a non-standard 30 amp outlet.
A UK type 240 volt kettle could be used in the USA from a 240 volt outlet, but these are non standard and not worth installing to boil water a bit quicker.