Thanks for the tip.
Could you explain what do you mean exactly by : "using the RGUI I assigned the enable Turbo function to the same button I want to apply the Turbo function"?
I cannot find how to do this in RetroArch.
So far I created the cfg file for the game :
input_player1_turbo_btn = 1
input_player2_turbo_btn = 1
And I put this code retroarch.cfg :
input_turbo_period = 6
input_turbo_duty_cycle = 3
another thing you can do is change the retroarch video driver to dispmanx which is said to save 1 frame off the input latency. i haven't personally done this as it has some side-effects (no shaders, no OSD, no screen rotate)