So I've been able to set up another green wall, with new models and shaders completely redone by me from scratches, without using any opacity map and without the Forest Edge mode.
They all have a front/back corona map in the diffuse, front/back corona map in the glossiness and a bump map. PBR mode on, no reflection map and thin (no refraction) tick on for all the leaves.
I was quite ok with the estimated time after 5 passes, which was 5 hours and a half, but as you can see from the screen-shot, it went up quite quickly to almost 9 hours for a target of 3% noise., and it was still going up.
How could it be that the estimated time get almost doubled after 20 minutes?
And, in any case, is it possible that all the time that I will have to deal with vegetation with Corona, I'm gonna end up with such big render times?