speed vs. practicality
To reduce most of the friction and therefor get maximum speed out of a bearing, we would remove all seals and most of the lubricant from the wheel bearing. If speed is your main objective, then go this route, as these two factors have the biggest impact.
However, how far would you be able to ride with these bearings before they wear and get damaged because of dirt and water, so that now they have more friction than your basic sealed ball bearing? It won't take long when using them in bicycle wheels as you can imagine. That is why for most riders it makes sense to have seals and a smooth lubricant.
steel vs. ceramic balls
A factor for quality bearing balls is roundness (or smoothness). This is graded by a number (lower is more round). Steel balls are around Grade 25, where as ZERØ ceramic balls are Grade 3.
However, this ball roundness factor should not be overrated. We also have to look at the material hardness and the quality of the races. Because without a good surface to run on, super smooth and hard balls will not make the difference or even speed up the process of wearing out the bearings.
Steel balls are hard, but (quality!) ceramic balls are much harder. Anything that gets stuck between the ball and the races will be grinded down to almost nothing, ensuring longevity and a continuous smooth rotation. When combined with hard races, the longevity is even enhanced.
Enduro ZERØ uses Chromium races which have gone through a series of treatments to make the races harder and tougher. Then the ZERØ races receive a Magnetite treatment (black) which is corrosion resistant and further stabilizes the bearing assembly. The races are again polished after this process to give the smoothest ball path. A perfect match for the Grade-3 ceramic balls.
Our standard setup for hubs has TPI bearings with 2 synthetic rubber seals. They are non-contact seals (LLB), meaning they have very low friction.
The Enduro ZERØ ceramic bearings have Silicone non-contact seals that are more slippery than the rubber ones.