@ xSINZx, he's talking about the matter fabricator (from GregTech), not the mass fabricator. The matter fab won't even run without scrap at all, and even with it, the energy consumption profile is completely different. On default settings you need 100 times the energy per scrap consumed than the basic mass fab.
@ Schpoon, The only way to improve the speed of the matter fab is to feed it with more energy. With 4 MFSUs sending to it, you have 512 x 4 = 2048 EU/t input. This should result in one piece of UU-Matter roughly every 6 minutes 47 seconds, provided you can keep that kind of input up at all times. If you had twice as much input, it would run twice as fast.
Basically, the only hope you have is spamming HV/ultimate solars. That's why I consider the default setting counterproductive (instead of nerfing solar-based UU-matter spam, it encourages it). In my worlds, I lower the matter fab's energy draw from 16.6 million down to 2 million, while halving the output of all solars and slightly upping the output of nuclear reactors. Feels much more sensible that way.