It's a bit unfair to compare the speed of Mitsuba with the speed of Renderman. Mitsuba only has a simple KD-Tree (although highly optimized), while RenderMan most likely has a state of the art BVH, which utilizes SIMD instructions. RenderMan also has a lot of sampling controls, while Mitsuba's path tracer has always traces one light and one BSDF sample on each intersection. There are multiple software engineers at Pixar that solely focus on optimizing RenderMan, while most of Mitsuba is implemented by a single person, who is a full-time researcher.