If you check out my line 64 on the BWB game you'll see two months where it hauled 525 units for ~18000 profit, the average speed registered is 3, despite the brigs going normally at 15 kmh.
Perhaps this is because some ships are waiting at the dock? Either way, seems like a bug to me. When I run the game offline in fast forward the registered speed is 15.
Does it always register this speed, or only occasionally? It would be helpful to have a saved game in which this can be reproduced where the ship is just about to dock and register this incorrect speed.
Can I check whether this bug still occurs or whether it has been fixed with fixes in 11.19?