Low fps until I oen and close quest menu

Hello, I'm building a quest 2 game in Unity that runs at 90fps. 

I'm experiencing a strange behaviour, when I run the the apk, the games average frame rate is 71fps (I think it alternate between 90 and 45) and stays like that for an indetermined time, sometimes it goes to 90 after a while, but everytime if I open the quest menu (by clicking the oculus button onthe right controller) and close it, it goes to 90 and stay stable after that.

I really don't see how is it possible.

If someone have an idea.

Thanks for any help.