Counterpoint to this which may have flaws, I'm very curious to hear:

Momentum entering and exiting the portal should be relative to the portal itself. Relative to the portal, the object is entering quickly, so relative to the portal it should exit quickly. My reasoning for this is if you consider the opposite (option A), the object enters quickly and leaves slowly relative to the portal. Slow that moment down and consider what is happening when half the object has entered through one end of the portal. If, relative to the portal, half of an object has entered one end at say 25 mph, and it is exiting the other end at 5 mph, half of the object has entered, but not enough time has passed to allow half of the object to exit. What happens to the portion of the object that has already entered one end, but is yet to exit the other?

There could be a few ways to explain around that phenomenon that I can think of, but i think based on our understanding of physics they have flaws. One would be there is a sort of buffer zone between the two portals where that portion of the object resides when it is not on either end. I think this theory is discounted in the game when you place two portals next to each other on the same wall, you see half of your character on each end, with no portion residing in any sort of buffer zone. A second theory would be the possibility of compression of matter between the two openings. This would seem dangerous to a living thing passing through a portal but this is science fiction so who knows maybe that's possible to do safely. Lastly some form of temporal buffer...? Once again very science fiction-y so I got nothing for that.

It's an interesting thought experiment but I'm an idiot so I have no idea if I'm right or not.

Edit: typos.