Path Guiding

Works with Path Tracing, on the CPU or on the GPU with Metal, CUDA or OptiX, and switches off while Photon Caustics is on, where it only added render time in my tests. Render Properties → Sampling → Path Guiding; tick the checkbox in the panel header. Guiding learns during the first samples where the light in a scene comes from and aims paths there.

It is not a general speed-up, in Reblend or in Blender. It pays off only when both are true: most of the light reaching what you see has bounced first, and that bounced light comes from a small part of the surroundings, such as a bright patch, a gap or an opening, which random bounces rarely hit. Otherwise there is nothing for it to learn, and the learning and the extra work per sample make the render slower. That is why it is off until you turn it on.

• Does not help where the lights shine directly on what you see, or where bounced light comes evenly from all around.
• Not for other noise: caustics (use Photon Caustics or Bidirectional), depth of field, motion blur or fine textures.
• Check your scene: render it for the same time, not the same samples, with guiding off and on, and keep whichever is less noisy. A guided sample costs more, so a comparison at equal samples flatters guiding.
• On a Mac, tick only the GPU in Preferences → System when you render with Path Guiding. With the CPU ticked as well, renders on my M1 Pro came out noisier in the same time.
• Training Samples: how many samples are spent learning; 0 keeps learning until the last sample.
• Photon Training (GPU only): gives guiding a better start, at about a second per render.
• Surface and Volume switch guiding on or off for each.