Sometimes When I charge my phone it seems to charge super slow. Sometimes it doesn't.

I had a hunch it was either the USB port or the USB extension cable. I tested using a Voltmeter device I got from GearBest.com they called a USB Voltage & Current Detector with Load Resistor.

I was provided with a free sample to write a review. I specifically inquired because it was a product I was interested in.

Device	No Load Volts	Load Volts	Load Amps
USB2 Rosewill Internal 2.5" Multimedia Reader	4.9	3.48	1.3
USB2 Mobo to Case	4.98	4.4	1.66
USB3 Mobo to Case	4.98	4.31	1.56
USB2 Power Extension Mobo Through	4.95	3.07	1.15
USB2 Power Mobo Direct	4.97	4.06	1.73

I put the Google chart here:
https://docs.google.com/spreadsheets/d/1CGjlTdy6c7uFeEc0WRNjKz_Z84ZOzBsOfk4osGu36Mc/pubhtml

To sum it up, the USB 3 actually seemed to put out a bit less power to my device, but not much. The biggest impact on power was using a USB extension. It significantly decreased the voltage. However, it is worth nothing I recently tried using another USB extension cord of the same length, yet somehow delivered more power. So, the quality or type of USB cord may differ. The better performing one was actually older, but had gold plated connectors.

Let me know if this data has helped you!

I should warn you that the USB voltmeter resistance battery (green tubes) seemed to get pretty hot pretty quick. I left it plugged in for about 10 seconds each time to let the current stabilize, because it fluctuated the first few seconds each time. The hot temperature is to be expected according to the website linked at the time, but since I was putting it through many tests, I didn't want to stress it.