Re: how do you shut down apps on the htc1 m8?
Android is designed to leave apps open and dormant in the background. They don't use any juice unless doing something like syncing data on a schedule, so charging times wouldn't be affected by that. In general, you're best to not manually force close them most of the time.
Your problem is most likely a bad USB cable, as they are prone to going bad and I've yet to find one that I'd consider durable. So the cheapest option would be to try a new cable first. If that doesn't work, it could also be the charger or even the port in the phone itself. There is an outside chance that it's the wall outlet, but I've only seen that be the issue once (which was a very old one in a non-ventilated bathroom, which could be corroded).