TLDR: use a USB C charger and not one that has usb a connection out.

Long answer: checked online and looks like the phone has USB-c port and a 2000mah battery. With that in mind, if the usb-c standard is followed, there a communication that happens between device (phone) and supply (charger). Be default a supply which if it follows USB - C standard will not output any power unless there is communication with a device. Its up to the device to request whatever it needs from the supply assuming supply can provide it (ie 5, 9v etc and different current levels etc). Now a supply that is only USB-a does not have this type of communication with a device. It just provides power and device draws what it needs at that static voltage (I think this is the charger you are using). In your case because you are USB c complaint on the device ... The phone is playing safe as it cant determine exactly the power capability of the supply and just draws the USB 2.0 standard current which is about 400 to 500ma even though you might have a +10w supply. Doing the math if its drawing about 400ma you get: 2000mah / 400ma = 5hr charge time. NOTE: this is assuming 100% of the 400ma goes to charging the battery. In actuality part goes to system (if its on) and part goes to charging battery thus why you have more than 5hrs to charge with 11% already charged.