Quote from Kryyn
A lot of the reason they are laggy is because it's a mod without a source code or a developer's kit, and that minecraft itself isn't very optimized.
I don't really care for the discussion, but just throwing that out there. If there was a dev kit, the mods would work much better.
Or if Notch just incorporates them (and of course, puts his own spin on it). I find Millenaire to be well thought out and well implemented, with villagers slowly building houses that take the terrain into consideration. Notch's npc villiages probably look much like what Millenaire started out as in it's beginning stages.