My kettle boils a mug's worth of water in less than a minute, and it takes me longer than that for even a brief toilet visit and washing of hands. I have learned not to switch the kettle on until I get back from the bathroom, otherwise I'll be boiling the water twice.

Important factors: 1) Britain has 230V mains power so electric kettles can boil water incredibly quickly, 2) The stereotype about Brits and tea is true in my case. I get through three to six mugs of the stuff per day. 3) Hot tea must be made with boiling water. Power isn't cheap and re-boiling the water adds up over time.