He's not super athletic. (Compared to other NBA players, obviously)
He doesn't jump very high and he's not quick. No fluidity to any of his movements either. He also doesn't really play with much strength, regardless of how muscular he looks.
For a center he is pretty athletic. Here are some combine results for centers from recent years:
PLAYER, POS, LANE AGILITY TIME , SHUTTLE RUN, THREE QUARTER SPRINT, STANDING VERTICAL LEAP, MAX VERTICAL LEAP, MAX BENCH PRESS
Steven Adams C 11.85 3.02 3.40 28.5 33.0 16
Rudy Gobert C 12.85 3.19 3.57 25.0 29.0 7
Meyers Leonard C 11.34 - 3.41 30.0 32.5 19
Jarrett Allen C 11.82 3.00 3.21 31.5 35.5 4
Moses Brown C 11.95 3.30 3.51 27.0 31.0 -
DeAndre Jordan C 12.30 - 3.27 26.0 30.5 8
D.Cousins C 11.40 - 3.55 23.5 27.5 -
H. Whiteside C-PF 11.83 - 3.54 27.0 31.5 12
I would agree that he is not fluid on the basketball court, but he sure is agile and fast for a center in the raw "athletic" measures.
At the end of the day - I still believe he will be in the league for a while, he is athletic, he can shoot the 3, he is not a problem teammate. Someone will pay him, even if it will be a minimum.