My friend develops PixelCNC, an OpenGL app for generating 3D tool paths from 2D images, on a netbook because he knows many of his users have under-powered older systems.

This is so true. My workstation was i5 2nd generation with my development code running on a partition which was on HDD, the timings for every request were ~200ms to 1s depending on multiple factors. Page performance in chrome was constantly ~1s total.

I upgraded to latest i5 with SSD for data, and the performance drops are basically 0. It bothers me that there is something which needs improvement and I cant check on a commit to commit basis. But yes, it is very easy to oversee performance drops due to new / faster hardware.