yeah, PBRT is in an interesting place when it comes to changes and optimization. They have an explicit goal of keeping the current released repository as close

yeah, PBRT is in an interesting place when it comes to changes and optimization. They have an explicit goal of keeping the current released repository as close to the book ("Physically-Based Rendering") as possible, so all of the optimizations and changes go into -next. Matt's blog talks a _lot_ about all the optimizations he had to do just to get the scene to _load_ in PBRT. It's a beast.