This was new to me, never read about it either, when I noticed I was producing units in one turn that normally take two turns. I figured out that having retinues that give bonuses to reduce unit cost ALSO reduce unit production time. I noticed that adding retinue until I reached 50% discount on cost also reduced the build time for two turn units down to one turn.
Tested this with a 40% discount. The time returned to two turns to produce. I do not know if the ships taking three or four turns will produce faster with less than 50% but can test this out tonight. I expect the production time is modified by the discount with fractions rounded up.
I found this out playing Seleucids. I have Antioch making a bronze chevron/silver shield/silver sword Cataphract every turn. I am currently using Rome: Total Realism 4.1, large units, hard battles, and very hard campaign.
If interested, I can check which retinue is giving the bonuses. I know I have a priest that gives 10%, mercenary captain 10%, a Hero for 20%, and (I think) the drill master for 10%.
I feel like the last person in the world to realize this. Might even be in the game manual if I had taken time to read it. (My first thread here!)
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