My iPhone 5 charger declining?

I've noticed that my iPhone 5 (well, all 5s) charge considerably fast like within an hour or a little over. My relative asked if I can charge her 5 so I did but it didn't seem to charge it as efficiently, nor seem like it charged at all and after a couple hours it went only from 5% to 19% so I thought the charger was like declining in terms of usage. But now I plug in my 5 again and it shows that it's charging but it's taking noticeably longer to charge my 5 and was only about halfway charged after an hour. Does the cable get affected if you charge several different phone through it? Will I need to get it replaced? And has it technically really broke because of what I did?

You can try the charger in another iPhone. I think it is not the problem of iPhone charger, it must be the problem of your iPhone battery.