actually that the why they are slower. most plug in devices in the usa are limited to 1.5 Kw. weather you used a 120v or 240v current it would just change how many amps it draws
2 kW @ 120 V is 16.7 A, which exceeds the 15 A limit on most household wiring in North America. To be able to achieve that you’d need to get a 20 A rated circuit installed by an electrician which means pulling out and replacing the wiring with a heavier gauge.
The advantage of 240 V rating in the UK is that you can draw more power with less current, so you don’t need the wiring to be so heavy for a high power appliance like a kettle.
Something rated for so few wouldn't be a good heater. At 1.5 kw that typical cut off for small devices in USA. You draw 12 amp. Plug it into a larger 240 vlot circuit and it draws amps but you still only get 1.5 kw of heating. Same time to boil water