[Suicide Junkie]

Retrofits can be done once per turn in a simultaneous game (such as PBW)

Since the cost can be 50% more, you will save some time as soon as the cost of the vehicle exceeds 3 times your spaceyard rate.

Of course, that is just for one ship.
If you want to build a fleet, you are in a better position.
Say you have a retroseries with the first ship buildable in 1 turn, and 5 retrofits later you get the design you want.
One factory can build the design you want in 8 turns. (5 retrofits = 7.6 times the value of the original ship)
Or, one factory can build 8 retroseries hulls in 8 turns, and have three of the vehicles fully built by that time. From then on, your factory produces one finished ship per turn instead of one every 8 turns.
You will be spending a lot more resources to do the retrofitting, but the speed gain is immense.