How to speed up ship construction?

[Vanigo]

Yes, there are two relevant tech lines. Improving your shipyard construction rate improves your shipyard modification rate by the same amount. Also, there's a "reduce shipyard modification time/cost" tech line, which doesn't actually have that much of an impact.

[jRides]

The bigger a shipyard gets, the more construction capacity it has, so it builds faster (it doesn't take 100 times longer to build a 100kton ship than it would a 1kton ship) an additional effect of that mechanic is that expansion actually gets quicker, the bigger it is.

[UnLimiTeD]

With max tech, did that for a test, you can modify your yard with economy-breaking speed.
Had a single yard with 3 1000ton slipways.
Adding a slipway took less than a month, and after 4 months and 5 more slipways, I was bankrupt.