Thanks for finding this!
I couldn’t imagine a threshold so high and stopped investigating further.
Changing glossy roughness to 0.5 in my scene (also increased exposure to 4) I now get results:
PG off, 149s:
PG roughness-based, 228sec:
PG RIS, 407s:
@sherholz : With glossy guiding working so well at 0.5, maybe the threshold should be lower than 0.45?
Or maybe some smooth fading out of guiding probability towards lower roughness, with regard to roughness maps with values differing all over the place?