The various factors taht determine USB cable charging rate are:
-The size of the power wires inside the USB cable determines the amount of power the USB cable can carry,for example a standard cable is rated 28 gauge and is capable of 0.5 Amperes. However other sizes exist like 24 gauge which is capable of 2 amperes or more. The amount of current a wire can transfer is limited by its size or gauge.
-The length of the cable will also affect its rate of charging , longer cables will create greater current flow resistance which reduces the cables ability to charge faster.
-the quality of connectors in the USB will also affect current flow, if they are of poor quality ,rusted or broken this will reduce their ability conduct electricity at full capacity.this may be caused by accumulated stress or wear due to usage over time.