There are many types of USB devices on the market and even more brands of cables on the market.
Not all USB devices are capable of charging at high speed. The speed at which your USB device can charge is determined by the USB device you are charging.
If your device can handle high speed charging, it will draw that, however many devices are restricted to 1.0A input.
The USB cable being used is also a factor. Certain cables cannot handle high speed charging.
If you do not feel your device is charging as fast as it could, we suggest trying a few different USB cables to see if that might be the issue.