I see that most people here are testing scenes with glass or glass-like materials that are impossible in standard Cycles. What is the expected performance for more simple scenes?
I tried the build on a few of my scenes and it took me quite a bit of effort to find one where pathguiding performed (marginally) better for the same samplecount (but longer render time). Usually it was worse for the same spp and definitely worse for the same render time.
The scenes generally contain no (or very little) transmission materials, all principled shaders and a mix of environment light and a few area lights.
The only scene where I got slightly better performance for the same sample count was a simple tiny room with a few objects and environment light coming through a hole in the wall (no other lights). When the room was sealed (no light from outside) and lit by a single area light, the result of path guiding was worse.
The scene follows the guides cited in the first post, IF the first point means that it can be a principled shader material and not a literal mix of the basic diffuse shader with something. Otherwise this is probably the reason for bad results.
Pathguiding on, 40spp, 2m 12s:
Pathguiding off, 40spp, 1m 12s:
The no pathguiding version is slightly noisier with more prominent patterns in the noise, but even here the difference isn’t huge.