I've noticed that when I try and charge my iPhone off of the USB port, it can take four times longer to charge compared to charging off of my regular laptop or off the wall charger. Does anyone have an idea why this would be?
Yes the delayed charging happens regardless of the Surface being plugged in or running on battery. I have a full sized laptop that charges from 0 to 100% in (guessing) 2 hours. On the Surface two hours isn't getting me 25% of charge.
Sounds like you laptop supports fast charging. USB 2 is 5V .5A, USB 3 is 5V .9A most cheap chargers are 5V 1.5A. Larger devices have chargers that hit 2.1A, enabling Fast Charging on many phones. With all that, since the Surface is designed to be a bit more mobile than a laptop, it's probably withing the 5V .9A range period.