"The first thing to ask is where the speed advantage comes from. It comes from reduced rolling resistance, increased durability, increased stiffness and reduced weight.
Rolling resistance is reduced because the balls are generally ground smoother, rounder and more uniform in size than steel, so they are each carrying close to the same load, rather than larger balls carrying more than smaller ones within the same set. And unlike aerodynamic friction, which becomes more of a drag as riding speed increases, mechanical friction is linear.
Because ceramic bearings are harder than steel bearings, they are more durable, lasting anywhere from five to 20 times longer. This also equals an almost zero risk of bearing seizure. Consequently, since they also do not rust, they are less sensitive to moisture and lubrication, and some even come with no lubricant at all.
Ceramic balls and races are also stiffer than steel ones, resulting in less flex in the bottom bracket. Finally, ceramic bearings are lighter, because ceramic is less dense than steel (silicon nitride, for instance, has a 60 percent lower density than steel). When combined with special seals and optimized lubrication, the friction in ceramic bearings can be 20- to 30-times lower than steel bearings with standard seals and lubrication."
