I just tested, and discovered something weird.
When using original HTC charger, and original HTC microUSB cable, the charging current is 1A, but when I use USB extension cable or spare microUSB cable charging current is 0.5A (490mA) using same original HTC charger.
What's wrong with the USB extension cable or spare microUSB cable?
Original charger and original cable
Original charger, original cable through USB extension cable
Original charger spare microUSB cable
(spare microUSB cable is 1.6m long)
HTC ONE M8