Ok, so I investigated more on the problem, and it seems to be caused by the fact that I cannot access my spawned character from my controller!
Basically, my character knows it has a controller, but my controller doesn’t know about his Character, so when I cast my character from my controller, it will always fail on beginplay… My drop in FPS is due to my controller trying to access my character ( who is a stored variable declared at begin play… )
EDIT: I just solved the problem: for whatever reason, casting to my character form my controller on begin play doesnt work anymore on begin play… so I just added a delay node as the very first instruction of my begin play, and now it works… Dirty, but it will have to do…