You can pre-build components for the ships that you plan to build. If you have a stockpile of pre-built components, then when you lay down the new ship in your shipyard, the construction time will be significantly reduced. The actual time-saving depends on several factors, but can be as high as -75%... perhaps even more. Not all components can be pre-built, because some ship systems need to be specifically tailored to the shape and function of your ship, and can only be built as the ship is under construction. Cargo Holds and Fuel Tanks need to be form-fitted to the ship's contours, for example, and Life Support is integral to the entire ship (ie: spread out throughout the ship's volume). Generally, the only components that can be pre-built are those that are modular and localized... missile launchers, tractor beams, engines, jump drives, etc.