AssetBundle.LoadAssetAsync case freezing in Unity5.5.0p1

When called AssetBundle.LoadAssetAsync many times in the same time will case editor freezing. (android is the same problem)

Like this:
AssetBundle ab = xxx;
AssetBundleRequest req1 = ab.LoadAssetAsync("a1", typeof(GameObject))
AssetBundleRequest req2 = ab.LoadAssetAsync("a2", typeof(GameObject))
AssetBundleRequest req3 = ab.LoadAssetAsync("a3", typeof(GameObject))

Called many times in the same time, and then case freezing.
If called from coroutine one by one will ok.

Is anyone has this problem?
My project is upgrade from unity5.4.3 to unity5.5.0f1(or 5.5.0p1)