You presume too much about those your correspond with- it isn't attractive, or helpful. Others have explored similar 5 axis avenues built around aluminum extrusions, with a degree of success. Mindful of material constraints, the spindle and BC axis are kept exceedingly light, delivering reasonable speeds (5Axismaker), or burden the assembly with more power (PDJ), limiting speed, as you indicated. I reckoned this circumstance was implicit in the material suggestions made in my previous post.
There is plenty of good information on this venue; as with anything in life, finding value requires discernment and discrimination.Never believe what somebody posts in here.