You know my villagers at times seem to go pretty fast at getting the first two mastered, but they seem to slow down at the third one. It seems like they stay at adept a lot longer than the first two. And I keep my speed at 2x all the time except when I am not returning along time then I pause it.
I myself seem to think that as they get past the first two they are usually a lot older therefore they tend to slow down.