But when I added the scene to the view it tanked again.
Probably because the visualization of the QGraphicsPathItem still involves mapping those mmillions of segments into graphics calls. Doesn't look like that class is optimized to eliminate redundant operations.

You could look into how QGraphicsPathItem does its painting. That is most certainly at the pixel level, so you may be able to derive from this class and implement the optimizations there prior to jumping into the OpenGL blender.

You could also look at how level of detail is implemented in the Qt 40000 chips example.