What is the difference between USB charging and AC charging?
In fact the charging time depends on two points: 


the output current of the adapter or the USB. The output current of the PC is only 0.5a, it may charge slowly than most of the AC adapters (most of the AC Adapters are 1A or more).
the input current and the charging convert rate of the phone. Different phones have different input current due to Limited current protection for them. For example, the limites current for Nokia is 0.5a, the input current is only 0.5a even you use an ac adapter with 1.0a output. Beside, the charging convert rate is also different according to the models.