To go from 1.000t, 1 slipway to say, 30.000t, 4 slipways should take the same amount of time, no matter which way you go (If not, I´d consider this a bug).
But, as Brian says, if you expand capacity first, you can start building ships as soon as you hit the size you want and then start adding slipways, while if you do it the other way around, you have to wait for the whole process to finish before you can lay down the first ship.