How do I quickly select all the photos in an album in Google Photos iOS app?
Selecting a large number of images in an album could be quite cumbersome. You can greatly speed up the process by employing a neat trick built into iOS.

Once you are into selection mode, tap down to select an image. Now without lifting your finger, simply drag your finger in the direction of unselected images (i.e. towards the bottom of the grid of images). As your finger moves, the grid of images will automatically start scrolling, selecting the images encountered on the way. The further away you move your finger from the starting point, faster the selection will be.

The GIF linked below demonstrates the same in action:



This trick also works in the built-in Photos app, and greatly speeds up select large number of images.