Okay, so finally it was the Mobile HDR, you were right !
For the AA, I didn’t know that Gear VR has a “Built-in AA” and that I was able to let “none” in the AA settings (in rendering). Still I enabled r.MobileMSAA = 1 and r.MobileOnChipMSAA = 1. It work perfectly, even if I loose a bit of quality on reflections, but the FPS are smoother.
Thank you guys for your answers, and Happy developing !