You should avoid using your device while it is charging. This is because a significant amount of battery is used to power the use of the phone at the same time, for example, when you use the Facebook application or play one of your favorite games. Therefore, do not use your phone if you want the fast charge to work as expected.
Many LG G5 fast charging problems arise from improper use of the charging cable and charger. Remember to use a cable or charger that supports Quick Charge 3.0. The best thing to do is to use the cable and the charge that come as part of the accessories for the phone. If you select a different cable or charger, you will not get the expected results and fast charging will not work.
Take a look at the charging port to see if there is dust or dirt. If present, use a small needle and clean it carefully. See if the charger you are using has any damage. Try using another charger that supports Quick Charge 3.0.
The fast loading problems bothered many Android users in the past, and one of the things that turned out to be a problem was the fact that the USB cable was not fully inserted. Be sure to push the cable correctly, to the bottom of the charger and, hopefully, this will solve the problem.
Sometimes, a third-party application may be responsible for the problem. If the fast loading problems occurred right after installing a third-party application, uninstall that application because that may solve the problem.
If you tried the above methods, but still have problems with fast charging of LG G5, take your device and your charger to the store where you bought it or to your provider.