What contributes to the smoothness of the zipp hubs?

hi all, I have a set of zipp 404s… and I have a question

what exactly contributes to the smoothness of the zipp hubs?
the hub design? or the bearings?

that said, if my stock zipp bearings are worn, and i replace them with generic bearings that my LBS charges about 20 dollars for… will i lose the hub smoothness? or is the hub design the most important part of how good the hubs are

thanks!!