For what ever reason NPCs are much better at this than the Player. I know in a couple of games my vassals convert them years a head of me.

It would be great if they made a Justicair or Prime Minister for your counsel. Someone who could add their values to the whole range and work on converting the population to your culture.

In Spain it seems one may change after 50 to 100 years. That may be like one county in 20 or 30 changes culture. It must be the slowest place to convert.