The version of Unity VaM 1.X is using is highly restricted as to what can run on side threads. Most of the Unity API is restricted from side thread access. Things have gotten a bit better in newer versions of Unity, but there are still some restrictions. Unity does provide some async functions, and even 1.X uses those. But some things like actually creating game objects after an asset is loaded all ends up on the main thread and is the cause of stutters during load.
I'll do the best I can to improve this in 2.X, but there may still be stutters during load of assets.