If you have tested that map, you will notice that the 1st time the spell is cast, it lags for a brief period. That is because the ability's data was not preloaded.
An analogy of preload is that imagine a map is a bedroom. The ability is for example a shirt. When the map first interacts with the ability (shirt), it has to be taken out from the closet. Then when its done, it is placed in a nearby location (lets say the bed). When the ability (shirt) is used again, the system (human) will no longer needs to go to the closet and looks for the closer place which is the bed. The brief lag is when the system goes to the closet.
So to preload an ability, just give that ability to any unit at Map Init (like putting the shirt in the bed) so that when the system used that ability, it will no longer lag for a brief moment (since the 'shirt' is already nearby)