And if the blue portal is at a different angle than the orange portal, then… well, how fast is the milky way traveling through the galaxy relative to the universe?

The cube does have a non zero speed, it just depends what frame of reference you are measuring it by.

The question is: Is it the velocity of the object relative to the portal that matters or the velocity of the object in some other frame of reference?

In the later case, we must ask how the portals at different angles don’t send objects flying off at the speed of the milkyway’s travel through the universe.

The answer that requires the least assumptions and justifications is B.

“I’m wearing this maid outfit ironically”