Sometimes when taking a photo, the Camera app can continue to re-focus itself which can add some lag between captures. Luckily, if your focal point isn't constantly moving or changing, you can lock the focus to minimize lag and ensure that the focus is locked on what you want it to be.
How the lock the focus in the iPhone and iPad Camera app
Launch the Camera app like you normally would to take a photo.
Press and hold on your focal point until you see an AE/AF Lock banner appear at the top of the screen.
Remove your finger from the screen and tap the shutter button when you're ready to take your photo.
Unlock the focus and exposure again at any time by tapping anywhere on the screen.