Mmmm delta should help, but think of it as a pooling rate, at better fps, better data to work with
The Slider / Ask for calibration is a nice one, many games do that to calibrate arms length automatically
Oh my bad, try webXR with your headset, at least in quest 2 game is working fine.
Yeah Godot XR Tools have a common shader loader, in which you put all your materials and shaders, then put the commonn shader facing the camera (all the things you want to preload need to be in the frustrum of the camera and it need to be visible), is not like a precompile but like a work-around to load all things before starting the real game. I used that, so the game doesn't lag so much when doing the things the first time