Why is it slow charging from USB compared to charging from the power plug?
In my opinion, it's maybe because the computer's motherboard limits the current when charging through the USB. 
In details, there are two phases when charging a battery. One is constant current phase, which is fast and powerful and happened at the beginning. The other one is constant voltage phase, which is slow and happened during the ending phase of charging, but this help to keep battery full.
So when charging via a charger, the optimized method is applied, which is const-current first and then const-voltage. However, when charging via USB, the computer limits its voltage to 5V, which forced a const-voltage phase.
This is only my experience and guess, and I'm glad to hear other answers from you brilliant guys.