In addition to providing some battery tips, the official Galaxy S7 user guide makes some disclaimers regarding Fast Charging, and one of them we found particularly interesting: for Fast Charging to work the screen must be off.

Here are some more tips and info on Galaxy S7 Fast Charging, directly from Samsung:

About Fast charging

• Fast charging uses battery charging technology that charges the battery faster by increasing the charging power. The device supports Samsung's Adaptive fast charging feature and Qualcomm Quick Charge 2.0.
• To use Fast charging on your device, connect it to a battery charger that supports Adaptive fast charging or Quick Charge 2.0.

Increasing the charging speed

• To increase the charging speed, turn the device or its screen off when you charge the battery.
• While charging the battery when the device is turned off, the 5 icon appears on the screen.
• You cannot use the fast charging feature when the screen is turned on. Turn off the screen to increase the charging speed.

If the fast charging feature is not activated, on the Home screen:

• Tap Apps —, Settings —, Battery, and then tap the Fast cable charging switch to activate it.
• You cannot use the built-in fast charging feature when you charge the battery using a standard battery charger.
• If the device heats up or the ambient air temperature rises, the charging speed may decrease automatically. This is a normal operating condition to prevent damage to the device.