Today I saw a link to an article about the Moore's law for 3D printing: http://3dprint.com/7543/3d-printing-moores-law/
Apparently, the speed of 3D printing doubles every 24 months. As long as the printing heads become cheaper, improving the speed should be fairly obvious, the same as the evolution of the dot-matrix printers: Instead of printing one "pixel" at a time, use enough printing heads and thus "pixels" to cover the whole width of the part being printed. Then you can print one whole layer in one pass. Then you can optimize further. Start for example with running this wide "row printing head" on an oval track for a higher speed, instead of moving it back and forth. Next, put a number of such row printers one after another and instead of moving the heads, move the part under them. Each row will be printing its layer, and you'll print the whole part in one pass. And instead of feeding the plastic wire that needs to be melted in the head for each "pixel", feed the already molten plastic, using the printing head only to turn the flow on and off.
It would be somewhat more difficult for printing the metal parts from powder but the same principle can apply.