On an iPhone 6s or 7, you can use 3D Touch on an item in Mail’s message list to peek inside without loading the entire message. Just press hard on a message summary to bring up a more complete preview of the message. Either let up to close the preview or press harder without lifting your finger to load the full message. You can also swipe up while previewing the message to reveal commands for Reply, Reply All, Forward, Mark, Notify Me, and Move Message. It’s faster than going back and forth repeatedly from the message list to individual messages.