I’ve used electric kettles in the US and Europe and they’re barely slower. I think the difference is overstated. It’s often used as the reason that fewer Americans have kettles but I think the real reason is just that Americans drink fewer hot beverages other than coffee, and most people have a machine for that.
While 120v is safer for shocks I think the greater safety reason for 240 is lower fire risk which is more common and dangerous that shocks in either system.