Here is the official mesh instancing playground demo with motion blur post-process added:
https://playground.babylonjs.com/#c2ynt9#27
The motion blur acts like the instances are always in motion. I’m guessing it uses the original mesh’s position as the previous one, and so all motion is relative to the distance between the instanced mesh and the original.
The end result is any use of mesh instancing with motion blur enabled causes the instanced meshes to always be blurry.
Hope that helps!