If speed is your only concern then its hard to do it faster than jumping. It's over in about two seconds. However, to be sure that it will work, you must jump from high enough (at least 100 feet) and you must land on a solid surface (such as concrete). Landing on anythng else (grass, soft soil, vehicle, person, etc.) could break your fall and just leave you seriously injured. And there is another important reason for taking great care not to land on a person, because you could seriously injure, or even kill them.
The other fast way is to put you head (especially your neck) on a railroad track before a fast freight train comes. (I don;t know how well that method would work with a light, suburban railway, such as you get in some countries. But for a standard, main line, and an ordinary, heavy train, it pretty much guarantees instant death.)