Having units built instantly would be realistic in terms of the time scale of the game, but it has gameplay problems, given that the core of the game is about management and planning. Don't need to plan for defense if you can just snap your fingers and create an army at any moment you need one.
Having extremely fast units, again, is realistic in the sense of time scale, but hurts gameplay in a turn-based game. If I can attack you before you even know an attack is coming, there's not much counter-play to that.
Having extremely fast units, again, is realistic in the sense of time scale, but hurts gameplay in a turn-based game. If I can attack you before you even know an attack is coming, there's not much counter-play to that.