Faster than a speeding bullet is a really vague
measure, and really means 'real fast.' To be accurate, most
modern rifle bullets travel at or over
Mach 1.5. This is about 1,141 MPH at sea level (rag on me, I know, I'm not doing
the math) on a
standard day.
Many things travel this fast. The SR-71 spyplane, for one. Satellites, for another. After all, velocity can only be measured with a frame of reference, and so many things in our universe are moving away from each other at enormous rates that it's almost pointless to use this phrase except with the assumed rider "...within the same frame of reference." And that sounds, well, silly.
It's entirely possible that this phrase was invented by the writers of the original Superman T.V. series, where it was part of the intro: "Look! Up in the sky! It's a bird! It's a plane! No, it's Superman! Yes, Superman! Faster than a speeding bullet! Able to leap tall buildings with a single bound!..." ...and so on.