I'm not an expert, but I do competitive shotgun shooting -- one thing that I find is essential is to... not blink, and have your eyes open.
Sounds funny, but it's part of my 'routine' as I wait for the clay disk to appear; after all the stance bits etc etc, when I'm ready, I'm making sure to open my eyes wide, defocus on where I expect the clay to 'appear' and then, call "pull".
Then stuff happens that I'm not entirely aware off for the next 1/2s -- well, until afterward. As soon as I 'see' a shadow of a clay everything happens and I'm a bit of a 'passenger' until the shot is away. You see the clay in slow motion as you mount the gun from your low holding position, while rotating, and when it's on your shoulder as you rotate you do adjustments 'just so' and shoot.
And it works a LOT better if you let the autopilot do it, if you try to 'measure' stuff, you'll miss, guaranteed.
Worst case is when the sun is in the way and you have to 'squint' and the really worst case is when the clay goes from shadow to sunlight (or the other way around) as you know your retina is going to be laggy.
