Caching of indirect light. It is very similar to the VRay light cache.
Here’s an example:
50 samples without PhotonGI, rendered in 3min 5sec on my notebook CPU:
50 samples with PhotonGI enabled, rendered in 2min 21sec (including 47sec of building the cache):
You can see that at the same sample count, the render is faster and less noisy.
(and it even contains a few more light bounces, because photons are shot from lights, which makes this something like a render done with a bidirectional engine).
@marcoG_ita Thank you for the feedback.