Charge iPhone faster using AC power
If you’re in a hurry to charge your iPhone, plug it into the wall outlet. Apparently you can charge your iPhone 23% faster by using the AC power adapter when compared to charging the device through a USB port, according to testing.
The tests which are mentioned on TUAW pertain to the iPhone 4 specifically, but from experience I can confirm that an AC wall adapter charges the iPhone 3 and 3GS quicker than USB as well.
Another observation I’ve made relates to the power output of particular USB ports. For instance, a 2010 MacBook Pro charges an iPhone a lot faster than my Hackintosh Netbook, but still slower than the wall outlet.
Bottom line: if your iPhone is charging slowly, plug it into the wall to speed it up!