Don't overlook the possible benefits of adding pure mass to your machine. Part of the battle is not only rigidity, but also to kill resonance. For example, concrete is great stuff for this. Choose larger extrusions so you can fill up the interior void with something that will help dampen resonant frequencies. Fasten the machine to a concrete base when its done.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)