I read in the AD&D 1e core rulebooks that each spell has a point at which it can be "paused". So that means that each spell is actually a ritual by default, and that the wizard actually casts 99% percent of each spell, repeating it for each copy of it they cast. They leave out the last few motions so that the spell can be finished, its effect taking place. The spell slots represent the wizard's amount of on-hand components he can find easily.

I read in the AD&D 1e core rulebooks that each spell has a point at which it can be "paused". So that means that each spell is actually a ritual by default, and that the wizard actually casts 99% percent of each spell, repeating it for each copy of it they cast. They leave out the last few motions so that the spell can be finished, its effect taking place. The spell slots represent the wizard's inner power, their ability to take the mental stress of the arcane equivalent of an inert bomb, to keep their spell paused until they wish to cast it.