Actually since its displacing the same exact space of air, the air around the portal would eventually start speeding up as well, creating an extreme air current drawing most of the air in that room through the portal, eventually create a tunnel of fast moving air, slowly but surely ensuring that the speed of the object will resume increasing after a long period.
So eventually, (Though it would take much longer than the standard human lifespan), it could speed the given objects up to the point of traveling at or faster than the speed of light, since they are theoretically receiving a single G of energy, all the time, forever.
Oh, the TBFP memories.
I played runescape all day, i was even LESS productive then you were.