Sweet!! It looks great.
Actually I'm not 100% on this but I think that's a fairly normal growth rate. It's the squamosa's, derasa's, and gigas's in particular that get really big really fast. Crocea and maxima's, not so much. I know the ones I've had the longest have grown but seriously probably not more than 1" in a few years. You can see new shell getting laid down every month but millimetre by millimetre.
At least that's been my experience
Maybe others have different experiences..