This reminds me of Al Jefferson early in his career. Not skill wise, but how slow they look. Jefferson looked so slow his first couple of seasons that I didn't think he would ever make it in the NBA. Then during his third season, he began figuring things out and improved dramatically during the course of the season. He didn't get faster, he just began to understand where he needed to be and what he needed to do. And suddenly he looked much quicker. It wasn't physical, it was mental.
The same may be the case with Zizic. The NBA SL plays much faster than Europe and is very hectic. Once Zizic gets some experience in the NBA and gets used to the speed, he might begin to look much quicker by positioning himself better. I'm not predicting this, but I'm not ready to write him off either. The summer league is awful way to evaluate big guys.