Simply a question of available power. Typical British kettles use 3kW (comfortably doable with 240V@13A); under US power (110V@15A) you can only manage about half that, which means waiting twice as long for your kettle to boil.

(American here) I recently upgraded from a 1kW to a 1.75kW kettle and thought it was a huge improvement. It does trip the circuit breaker if I try to run the toaster at the same time as the kettle though.

Oh, I thought US circuits would be spec'ed at higher amperages to make up for the lower voltage. How does that work for high power appliances then? I have a 10kW induction stove, how could you ever get that kind of power from an installation running at 110V?

Stoves, ovens and so on are usually on a different power line from your ordinary socket.

They often run on 220v too.