Thank you @BananableOffense ,
I punctually use clear/reset the history, when passing from day to night, for example, as lumen keeps some surfaces shinning like emissives. But making it every frame destroys the lighting quality and causes a lot of flickering, as you already know, but for others, too.
About temporal accumulation is one of the mitigations I thought of, but as you pointed, could cause more noise, while Lumen is not the most noise-free lighting system. In fact I rised the accumulated frames to 15, as Archviz is a ‘slow game’, but this changed the whole concept, needing to be treated like a lightning FPS. I tried with a value of 3, which caused huge inconsistencies. Also with 5, with a slightly better result, but both cuased fast flickering too, making it VERY noticeable. 8 seemed like the minimum acceptable value, suffering some noise.
Ray Reconstruction is the best solution for now (I also implemented it), but it doesn’t solve it totally, and at a high performance cost.
So… I finally decided to reduce the fans speed at a very minimum, like blown by a soft breeze… but this also blow my mind.