How fast should I expect to render the silhouette of a 20k triangle model?
OpenGL (scanline rendering) is always going to be way faster than ray tracing.

Ray tracing computes pixels one by one using physics (at least a good 90% of the time it's going to be real physics). Although some of it can now be done with a GPU, it is not really helping as much as one would expect. Calculating speculative lightning is not something you can easily do in parallel (ray 1 will bounce from object A to object B, ray 2 will bounce from object A to object C... you can't parallelize the computations on B and C.) The GPU helps with the matrix math, though.

In comparison, OpenGL is a total fake in comparison. It renders triangles really fast with shades coming from textures and fast gradient computations from your various light sources.