X-15 is an American fighter plane. Its maximum speed is around 7200 Km/hr. Apparently, they are working on even faster planes. 7200 Km/hr means you can travel from New Delhi India to London in less than an hour. New Delho to London is around 6700 Kilometers.
By the way, you should also know the speed of sound – 1235 Km/Hr.
Another example would be to travel from London to NewYork in 45 minutes. In a normal flight it takes around 8 hrs.