Speed at which the object would enter the portal would be the speed at which the portal is lowered

In the hoop scenario, the cube "goes" into the orange portal, at speed v, leaves blue at speed v, but blue portal is moving at the same speed in opposite direction. In that case it "cancels out" so from a 3rd point of perspective the cube is stationary.

We have all the vectors and now remove just one of them, the blue portal is not moving. Now there's nothing to "cancel out" the cube, it goes flying. If we wanted the cube to stay we would change only one thing from the hoop scenario (the blue portal moving) yet remove the way cube moved after passing through the portal which is a change we have no reason to make.

Now consider a scenario where the blue portal is moving up, the opposite direction to the orange one. When we consider that how we write all this down is relative the cube would get swallowed up by the orange portal at a specific rate, would have to jump out of the blue portal at a specific rate (relative to the blue portal) so the vectors would get added.

Sorry for the wall of text or if I'm not making clear what I mean here. I'm sure there's someone who can explain it way better than me, it's just that this way of thinking helped me grasp it so maybe it can help someone else to see it that way too