Left trigger activates your precise aim. If the gun has a scope you’ll see the zoom window, if not you’ll just see the crosshair get more narrow. You can also disable the zoom window if you don’t like it. For actual iron sights you would have to physically look down the sights of the gun.
I don’t think the zoom window is actually low framerate. I can take a shot and see the projectile fly smoothly all the way. I think the actual issue is that it amplifies your natural hand shaking, as well as any tracking jitter. I disabled the zoom window and just don’t use sniper rifles in VR for that reason.