[Q]

I use it only in the construction of very expensive ships (e.g. stellar manipulation ships/ ring-/spherworld components). Once the remaining time is 1.5 years or less, I switch to emergency build for the rest of the time and the construction time is reduced by 1/3. In this circumstances the following slow build time is usually not important.
The other situation is the urgent need for a base with a space yard component. The slow build time of the colony is then compensated by the new space yard in the base.