Yes, I also thought of the pole example. It would feel like a wave of gravity suddenly shifting around them. I do think it would be less dangerous than a direct collision at the same speed, but only by a little - the danger with sudden deceleration isn't the fact that it's sudden, but that it is almost always uneven, it rips things inside you apart or collapse them together. But the portal would be a very clean uniform wave of pressure moving over you, so it could still cause damage by compression over one dimension (in the direction of portal movement) but most other forms of impacts will be missing