I would bet one of the iPhone's most underutilized features is 3D Touch. Since it's hidden functionality, it's impossible to tell what responds to "deep" presses without actually deep pressing everything. Plus the gesture isn't as intuitive as others (like swipes, pinches, and taps).
When you figure out where 3D Touch is implemented, it can be extremely powerful. A quick win I've found is 3D Touch-ing app icons.
For me it's much faster to select one of the 3D Touch menu actions than wait for the app to load, then navigate there myself. Even better is when I get the info I need without ever opening the app. These are some of my favorites that speed up my day: