Rubber seals generate heat, and if the heat cannot dissipate quickly enough, it will build up.
A shielded bearing will run cooler, because it does not actually make firm contact with the inner race. But, it is not a dust/moisture proof either.
If you have one side of the bearing closed within a housing, then you might remove the seal on that side, to decrease friction somewhat.
Machine spindles and high speed electric motors do not have contact seals. Instead, they use a baffle system to keep the grease in and the dirt out. The baffle is called a labyrinth seal system, which is not something you buy, but something you build. A labyrinth is a pair of disks with interpositioned grooves, creating a "square wave" path which the grease/dirt/air must pass through. But the one disk is fastened to the rotating member and the opposing disk is part of the housing. They run close together, maybe .010" apart.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)