Rushing something in total is also waste, since you are wasting a whole turn of shields production of a city. Short-rushing should be done prior, you see what units you have of lower value to the actual unit you want and you short-rush to it.
Worker-10
Archer-20
Sword-30
City produces 10 shield per turn - Sword ready in 3 turns - short rush worker, sword ready in 2 turns or instead short rush archer, Sword ready in 1 turn (10 shields lacking in the box will be provided by city - 0% waste) that simple.
There are also other tricks/exploitish nature to be used, civil engineers add 2 shields to a city production pool for improvements only, there for, begin by short-rushing units every turn and switching tobuilding and then adding civil engineers on end of turn, at beginning of turn break into build sequence and switch back to a unit, the shields will carry over, then short rush another unit and switch back to a building, add the civil engineers again and do the same process over and over, its a bit time consuming but you´ll be producing like mad.