Me too. I've been trying to find out what's it about with the spots. The same that you are wondering here. I do not know where they come from.
Today I've been doing tests with my 6-core xeon - the best muscle I've got at home. I have not come to any resolution with these settings yet.
You might want to try all the different settings with GI. I love the GI, but there are so much variations with it.
Maybe we could change the GI setting to Quasi Monte Carlo. Or maybe Irradiance Cache (legacy).
I'll keep on torturing my single 6C-3,33GHz xeon. Fortunately I've got two of them at my service right now. But it's all so slow though.
I've been doing this one render for 5 and a half hour now, and it's gonna be without spots, I can tell that allready. (edit. 11 hours now)Quasi Monte Carlo
. On the other hand, I've got a render without those perky spots, done in a time that is merely a 7 minutes+, with the default low settings. It does have it's little quirks, but it's quite allright considering the time it took to render.
I will update my findings, when I do have more results, and maybe something to conclude on this matter. It might take another year though.
Please find attached my personal _spotty rendered_ image.