I didn't realize Sengun couldn't speak English very well.
IMO, that would increase his development time.
Just looking at how Luka was brought in and the time it took for him to get acclimated with the culture, and how to fit in socially, and it may look like the same thing with Sengun. Not to mention communicating with assistant coaches, understanding the playbook, and so forth. I suppose it would be hard to get information about how he is in the locker room, and if he isn't another Porzingas (toxic personality-type).