A even more extreme example for a more complex scene:
> ./target/release/examples/pest_test -i assets/scenes/art_gallery.pbrt
Before:
BVHAccel::recursive_build(...)
PT1028.495833637S seconds for building BVH ...
BVHAccel::flatten_bvh_tree(...)
PT0.018149381S seconds for flattening BVH ...
After:
BVHAccel::recursive_build(...)
PT1.867626813S seconds for building BVH ...
BVHAccel::flatten_bvh_tree(...)
PT0.018222583S seconds for flattening BVH ...
That's 17 minutes vs. 1.9 seconds ... Funny what changing a couple of lines can do 