I think the Animation timing has to do with the Apps being Ready. I.e While it is taking 0.5Sec of Animation switching, in that 0.5Sec it is trying to get the Apps Ready as if it was fully loaded. So user assume it really is Multi Task, When as a matter of fact it is resumed from a Pause State.

While this may not have worked, ( iPhone 5 still has a lag after the animation ), may be iPhone 5S could manage to do without the lag.