Think of KBD's mobility being a bit similar to Luc Mbah a Moute at this stage, or maybe Luol Deng when he started slowing down a bit. However, KBD is a pretty smart and crafty player and can use subtle hesitations and angles to get an advantage on defenders. He can then finish pretty well with some burst at the rim. In other words, if his role is at the 4, I don't think his mobility is an issue. If you want him to chase wings on the perimeter or lead a break, he's probably a bit limited, but he should be okay for a modern 4.
Maybe his athleticism is also a bit comparable to Marvin Williams, but KBD is a craftier and more skilled offensive player, both in mid-range and in the paint.