That's my first post so... hello everyone :)

I've got the following problem with STL's or Visualmill VM6.

I'm preparing my project in grayscale, then convert it in Artcam to the STL, after opening in VM it's rotating and working reeeeaaalllyyy slow. I've tried on different computers and still the same. Any idea why is it working so slow?

How fast is your PC, how much memory and what video card?

I've tried this on :

Athlon (barton) 2800+
1,5gb ram
GF 5600

and on :

Intel core2 duo E8400 (3ghz)
2gb ram
Radeon HD 4850

and there was only sightly difference...

It is all relative to your faceting (triangle count) settings.
Billions of triangles take a lot of CPU/GPU/memory power.
If you can live with a lesser resolution your computer will speed up.

Personally I prefer using igs input (for VM), but that likely is not an option for you when working with Art as opposed to CAD.