I doubt in depth character sliders take any more computing time than shadows or reflections. While I don't know how a MMO does it and my knowledge of the required programming skills are pretty bad, I'd wager that they just slap a very lenghty code somewhere to the character. When they appear in the vecinity, the system reads that code and models their face until they leave. This would be no different nor more lenghty than drawing armor, boots, gauntlets, helmets or a weapon. It would also not take considerable space in the server. It's just one more variable for the character. Like quests.
Don't worry about it because it's a non-issue. Shaders, reflections and textures is were all the resources will go.
Don't worry about it because it's a non-issue. Shaders, reflections and textures is were all the resources will go.