The S5 will report how much current it is pulling to charge. I recommend Charging Time. Open that app, then plug your phone into your charger. If it's not pulling at least 1200mA, unplug it and plug it back in. If it's still not pulling 1200mA, try a different cord. I've found cords that only allow my phone to pull 450mA or 700mA. You can pretty much throw those cords away (or use them for data transfers only). With the proper charger and cord, your phone should pull between 1200mA and 1800mA. Furthermore, your phone should still charge somewhat rapidly even using GPS with the screen on, as long as you don't have the brightness cranked all the way up.

I have this charger and can confirm my phone pulls 1800mA on the 2.1A port and 1200mA on the 1A port, with proper cords. I have found dual port chargers that claim 2.1A/1A but only put out 700mA, so it may be either your charger or your cords.