Is there a difference between charging an iPhone 4 by USB to doing it directly from the electrical network? I think it is quite obvious that USB takes longer to get a full charge, but the difference between one method and another exceeds 30 minutes, that is, charging an iPhone 4 by USB takes 23% longer.
If the iPhone 4 battery is completely depleted, the time required to turn it on again after using the mains charging method is 3 minutes and 11 seconds while using USB is 4 minutes and 49 seconds.
Source: iClarified