Dear sweet jesus….
If you can ride a singlespeed, then you can ride a geared bike faster.
In almost every singlespeed ride, there are points at which a harder gear will allow you to go faster….that bit when you spin out.
In almost every singlespeed ride, there are points at which an easier gear will allow you to climb using less energy… that bit when you are thrutching up a climb.
A geared bike can do EVERYTHING a singlespeed bike can do, only it can do it at a wider range of speeds, and in mountain biking terms, a wider range of gradients.
No, there is no percentage evidence available for my suppositions, but that doesn't make it untrue.
Suit yourself.