[Unity5.5.4.p1&p5] Resource.LoadAsync seems blocking the main thread in Android(In editor is fine)

var st = new Stopwatch();
st.Start();
Resources.LoadAsync("large_prefab");
Debug.log("Time: " + st.ElapsedMilliseconds);

In Editor(Win7): the "Time" is 0.
On Android devices, the "Time" is 5000+.
I think this is a bug. Even after loading sync, the time is still 5000+.

var st = new Stopwatch();
Resources.Load("large_prefab");
st.Start();
Resources.LoadAsync("large_prefab");
Debug.log("Time: " + st.ElapsedMilliseconds); // 5000+ms on android devices

 

I want to make a background-preloading system which will preload assets with Resources.LoadAsync when cpu idle, but the Resources.LoadAsync will cause lag on Android devices.
Any suggestion?