Why there is slow charging in Lumia 535 with original Microsoft Charger ??

I have bought Lumia 535 in January 2015, After well going for 10 to 12 month, Charging with original Microsoft Charger has been slower. It hardly charged 10 to 15% per hour.

I have also use other charger but same problems repeats.

I got checked battery, but there is no any battery issue.

Please suggest

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