Yup - the issue with the 328Jet is the wing. It's straight (no sweep) because it was designed as a turboprop even if they had the idea of hanging fans off it early in the program. Straight wings give you great lift but drags rises steeply as speed rises (hence why almost all other jets have swept wings). It's a bit faster than the prop, but not as fast as some of its RJ bretheren that have swept wings.