Eyesplits - when the camera intersects geometry. A combination of the near clipping plane and the renderer's inability create micropolygons is a sure fire way to increase you render time (or crash the render)

Bad geometry - lamina faces, faces with zero area, etc.

Character density. An OBJ file is a list of points. Which file will read faster  - one with 8000 points or one with 80,000 points?

Always remember - choose bucket sizes and shading rates that prevent the renderer from sending information to the next bucket. For example, if your bucket size is 16 and your shading rate is 1, then your ideal grid size will be 256 (that is bucketSize^2 / shadingRate).

Considering that many people use 32 pixels per bucket and a shading rate of 0.2, each grid contains 5120 micropolys. 5120 is not a power of two number (and each bucket is square), which means that for each following bucket, 1022 micropolys are carried over from previous buckets, compounding. Could this adversely affect render times? Certainly.

Nowadays I believe it is unrealistic to exclude raytracing from scenes. People understand that it carries overhead and we / they just deal with it. But making sure your geometry and camera placement play to the renderer's strength, your renders can become speedier.