The HTC One M9 no wonder is a beast of a phone and comes with brilliant specifications to back its quality. The device comes with a very strong battery unit but we have seen lots of users complaining of slow charging issue on the HTC One M9.
It may be the battery but there could be some external factors which could be the case of such a situation.
Take a look at the video below, which should help you get rid of some issues you may have with your HTC One M9.