[knaq]

It wouldn't have to be slow.

Print on the inside of a spinning drum. That way, you don't have to reverse direction. Print from a whole row of nozzles or lasers or whatever. That way, yet another dimension doesn't require reversing movement. That leaves just one axis if laying something down like an inkjet printer. If curing with a laser, the final axis can be handled by the laser simply crossing the needed distance.