The micro usb connector can be stressed as the phone is used with the cord plugged in, eventually leading to the metal part being loose in the housing so it can wiggle around, leading to fatigue of the connections, resulting in them having higher resistance than they otherwis3 would, reducing current handling ability. Usb cables should be replaced once the metal part of the small end is loose.
Above issue, or sharply bending/kinking the cable can break the connections entirely - if a data line goes, fast charging is lost since the charger cannot tell the device to pull more than 500mA, nor negotiate quick charging.
There is a huge variation in quality between brands, and it's not always clear which will be better - but all usb cables should be considered consumables