Basically I think that conversion is way too easy in the game. In about 50 years of playing as Kingdom of Syria (in TPatT mod) I have converted the whole modern Syria and Lebanon to the Christian faith. Egyptians couldn't convert Copts (Egyptian Christians) in more than 1000 years, same with Maronites in Lebanon (shouldn't Beirut be christian maronite?), so why I managed to do it during only couple of generations? Not to mention, many of my provinces are Frankish now. The only provinces that should be moderately easy to convert should be pagan as they were less attached to their belief. I think that Muslim and Christian provinces should be converted much more difficultly and the only way to quick it should be to expel old inhabitants or kill them.