He's certainly slow for a point guard, but for his size he's a pretty decent athlete. A year ago he measured (in terms of height, weight, wingspan, and reach) about the same size as Tristan did at the combine. He's far bigger than any point or shooting guard in DX's database, and he's as big as any SF not named Kevin Durant. Pretty much any player his size will look slow trying to play PG.
As for his rebounding, he has the highest rebound rate on his team, and in fact has a higher rebound rate than Tristan did in college. He just had a game-high 15 boards against Arizona...Aaron Gordon had the second most with 8. There are many players in the NBA who prove to be above-average rebounders in spite of mediocre athleticism, and I have little doubt he will be one of them.
He's definitely a tweener between the 3 and the 4 defensively. I haven't watched enough film of him to get a feel for how he would fare against SFs (again, he's obviously slow for a PG, but how slow is he by SF standards?). He would have the size advantage pretty much every night; it's just a question of if he's quick enough laterally to be more than a big traffic cone. He certainly has adequate size and quickness to defend PFs, but he would have to pack on a bit of muscle.
Overall, I guess I'll say I think his athleticism problems are overblown. People say UCLA "hides" him on defense because he's not guarding point guards, but really that's an unrealistic standard for a guy his size. And he makes enough athletic plays (
like this dunk the other day, sorry for the shitty footage) on both ends of the court that I think he can overcome his limitations.