I have about 30 species of Coryphantha, but as you say the seedlings grow slowly. I have been told by a well known grower of Coryphantha here in the UK, that they need a lot of room for roots. I didn't understand why until I visited Mexico and saw a number of Coryphantha species in habitat. Unlike most Mammillarias (which like to grow in rocky places), Coryphanthas seem to like to grow in more open, flat areas, often with quite a lot of grassy around.
These are generalisations, of course, but when I next repot my plants, I will give them good room for root growth.