If I charge it with usb, the phone charge is slow, so why is that?

A. 1

The difference between fast and slow charging is due to the amount of current.
For usb, the amount of current supplied is not very large.
That's why charging is slow.

Gentleman 9 years ago 0

OK

A. 2

usb is the port originally created for moving data.
It was inconvenient to attach power, such as plugging a cord into a USB storage device or plugging in a battery, so the USB port started to provide some power.
So USB charging is quite slow.

Gentleman 9 years ago 0

OK