Fast way to select and delete all photos on an iPhone?
Click 'Select'
Move your finger from the bottom-right most photo and move your photo up to the top left of screen (so the screen starts scrolling up and selecting photos)
Here's the important step, with your other hand, tap the top left of screen (above where your finger on your other hand is). It might take a few tries but it will eventually speed all the way to the top of your photos only taking 1-2 seconds, whilst selecting all the photos along the way.

It's a little tricky, but very much worth it.