I swim 1 Kilometer every 25 minutes with boardshorts VS 1 Kilometer ever 15 minutes with speedos, what to do?

Alright, if you are doing it for training purposes, I would wear the board shorts. It cause a lot of "drag" which makes your muscles work harder and increases your endurance. Even competitve swimmers wear "drag suits" (a baggy type of suit) over top of their speedo when they train. If you are in a race though, it would be best to go with the speedo only to reduce the drag and lower you times.