The toolpath is calculated from the original NURBS surface, and in this particular CAM package, you have options of the tolerance that the toolapth is calculated off of.. The polygonal model would be dictating the staight lines and breaks and the calcualtion gets very heavy fast, on a polygonal model... The NURBS surface is far more desirable...

A toolpath would choke on a multi-million polygonal count, where it would be a breeze on the same NURBS surface, to acheive the same "curvature resolution", at say, .0001.

I just made a simple curved NURBS surface, then created a polygonal model from it.. The 3dm file is 17.9 KB and the STL is 62.6 MB. The toolpath caluculation on the 2 was 4 seconds for the 3dm file and 4 minutes for the stl...