They're a little slower to boil here because of the lower voltage
Not so much the lower voltage in itself, but because the maximum power which can be drawn from a regular wall outlet is lower:
U.K. 240 volts x 13 amps = 3120 watts max.
U.S. 120 volts x 15 amps = 1800 watts max.
In fact there seem to be few modern U.K. kettles which approach 3kW like many of the older ones. 2kW seems to be about average these days.
You could use a British kettle in the U.S. with a change of plug if you have a convenient 240V 15A outlet somewhere. Most American homes do have 240V as well as 120V, but the outlets for it are only fitted in a few specific places where needed (range, dryer, larger air-con units, etc.).
