For a single mug microwaves are quicker in America. Potentially even for 2 mugs.

BUT in America kitchen appliances have a power limit (usually) of 1,500W. This is usually higher than a standard microwave (1,000W). And since an electric kettle in America is just a heat source in water, it's very efficient. So if you're regularly heating multiple mugs worth of water, or just boiling water for cooking often, an electric kettle is definitely better. They are also pretty cheap.

Now in Europe and the UK, electric kettles are faster since they can often be around 3,000W or higher. But that doesn't mean American kettles are useless. American kettles a way faster than heating water on the stove. And WAAAY faster than heating water on a gas stove.