Which is the World's most deadliest and fastest snake?

Black mambas are the fastest snakes having a speed  up to 12.5 miles per hour (20 kilometers per hour) and have been blamed for numerous human deaths. For  these reasons, the black mamba is widely considered as the world’s deadliest snake.  They are Africa’s longest venomous snake, reaching up to 14 feet (4.5 meters) in length. They get their name not from their skin color, which is olive or gray in color, but rather from the black color inside of their mouth, which is seen when they open there mouth when threatened. Black mambas are shy and will almost always try to escape.  However, when cornered, these snakes will raise their heads, sometimes with a third of their body off the ground, spread their cobra-like neck-flap, open their black mouths and hiss. Before the creation of black mamba antivenin, a bite from this deadliest serpent was 100 percent fatal, usually within about 20 minutes the victim use to die. Unfortunately, antivenin is still not widely available in the rural parts of the mamba’s range, and mamba-related deaths remain frequent.