If the box left the blue portal at a slower speed than it entered the yellow one, the top of the box would be moving slower than the bottom of the box, which would compact the box dramatically during the transition, but we can see that its size is unchanged and it is still square.
The top of the box is pushed rapidly out of the blue portal by the bottom of the box being thrust into the yellow portal. (The bottom of the box is pulled rapidly up into the air by the top of the box which is already moving quickly.)
Everyone else is wrong. (Ragebait.)
Speed of approach = speed of separation.
If the box left the blue portal at a slower speed than it entered the yellow one, the top of the box would be moving slower than the bottom of the box, which would compact the box dramatically during the transition, but we can see that its size is unchanged and it is still square.
The top of the box is pushed rapidly out of the blue portal by the bottom of the box being thrust into the yellow portal. (The bottom of the box is pulled rapidly up into the air by the top of the box which is already moving quickly.)