Aren't the physical shapes calculated once and for all? (if no physical mesh is provided)
They are cached for a region but there is no global cache for physics shapes, and it wouldn't be good to store them as assets, so we have some room for improvement there. I think they are passed across on a crossing (I forget) but if you rez a boat, hop in, and cross a border, it might not be complete yet depending on how complex the shape is (how many mesh parts aren't set to SHAPE_NONE). It is common for some of the "optimized" vehicles to have dozens
of complex mesh prims that do have an actual physics shape too (and hundreds that are set to SHAPE_NONE).