Effectively, thanks for sharing.
Finally I still have some fps drops after applying these tweaks.
I have decreased the scenery complexity and autogen to normal and
I installed Prio then set high priority to the p3d process and set the affinity mask thru this tools (cores 1, 2 and 3 ; core 0 is free).
Right now I've 40 to 60 fps (limited to 40) without any drops with the carenado hawker (pilot view, 60 to 80 on external views) on a busy photo scenery. Core 0 seems to be used by pro atc, asnext and ezca.
my cpu now loads to 70-80% each core is used.