Goodness me, I wasn't even aware of the added time, now I checked, and it seems right - in my current game a Modern Atomic era route to a destination 12 tiles away of an actual trip took 48 turns to complete, as min duration was 21+20=41, so the trader made two complete journeys 24 turns each for the total of 48>41. I thought there was something strange with the routes late game, but never bothered to check them past early eras, as the game UI gives no such information. And it is like that since GS, apparently. O, man...
I don't know as to the why, it kinda pushes you to seek further and further destinations, and as later turns represent less years per turn, it may be seen as some sort of scaling, or maybe just to diminish the number of decision spam. Late game there are plenty of insignificant clicks anyway, no need to add more for trade routes.
There's a mod in the Steam workshop,
Better Trade Screen, which supposedly gives more accurate information, I haven't used it in a while, but judging from the comments it works.