Well, if they can solve the problem of rebar, then buckling won't be an issue anymore, since rebar has proper strength from the get-go. I don't really see a slower machine being much better than a fast one. The overall size of the machine depends on what you fabricate, not how fast you go about it (within reasonable limits of concrete pouring).
If I were to make a product out of it, I'd have a 5 axis machine with switchable heads. One head with an extra axis or two that can put out, restrain, cut and spot-weld rebar. Another head that can print concrete. With a 5 axis machine you can trivially print concrete on the surface of rebar going in any orientation. Heck, if they use a mix with fast initial cure, they can do skin/infill just like plastic 3D printers do, except that the infill uses a less viscous mix that self-levels. This could dramatically speed it up, and you wouldn't need to print around every piece of rebar but only some trickier ones.
This could be very much a breakthrough technology, but it would need a bit of capital investment as those machines wouldn't be cheap. For very large constructions, instead of X-Y-Z linear actuators you would need a delta-style arm. Even a big one could be assembled on site and print an entire highway overpass in a week or two, starting with nothing but a hole in the ground.