I assume because they need to be recreated most of the time? As much as I understand it, they are created by taking a picture of a 3D character model, and this is optimized by caching already created ones.Why are those icons taking so long to load?
So it has to be something very unnecessary and time-consuming in creating that 3D stuff, like if they are not creating just the guy (to take his mugshot) but a whole scene around him. For each icon.
This is just an uninformed wild guess though, I wish they would say more about the technical details of the bugs they fix or try to fix, particularly recurring ones, like the sound of doom or this one.