Why would a USB cable give less power after being used for a while?
Since you have two cables to compare, one of which is performing badly, grab a multimeter and compare the following measurements between cables:


Resistance along the 5V conductor, from one connector pin to the corresponding pin on the other connector
Resistance along the ground conductor, measured as above
Resistance along the D+ conductor, measured as above
Resistance along the D- conductor, measured as above
Resistance between D+ and D-, measured at the USB A connector end (the connector that plugs into the power supply)
Resistance between D+ and D- on the phone connector end


Such a drastic reduction in charge current is most likely down to failed high charge current negotiation, normally achieved by either shorting D+ and D- or setting particular voltages on them. Connector corrosion or partial screen detachment is unlikely to have caused such a large drop without having disconnected entirely.