There's a reason you don't often see machines over 300x300x400. At that point it gets hard to keep tolerances tight, requiring manufacturing changes or else you end up with printing artifacts.
This thing prints at 300mm/s at 1100x1100x820 and it's manufactured in a first world nation at low volumes.
It's hard to see, but I think they made the gantry (the whole Z platform, I mean) out of two plates of aluminum. They didn't bolt i beams together, it's just two massive plates with holes cut into them. That's the sort of engineering they did to get this thing to work at that size, with that speed.