Fundamental Difference Between Charging An iPhone At The Mains & Via USB Cable Linked To An iMac? If you are working at an iMac and there is also a mains socket close at hand, is there any difference (in any respect) to charging an iPhone via USB cable linked to the Mac or is it more advisable to charge via a charger to the mains socket.

eg

Is there a difference in the time taken to charge using one method than the other. ?

If using the Mac to charge, does it cause the Mac to use more RAM in the process (and could this prove to be detrimental on Mac performance if using resource hungry software). ?
According to Apple's support document on the subject:


  For the quickest charge, connect the device to a power outlet using the USB cable that came with the device and an Apple USB power adapter.


The USB spec limits the amount of current that a device may draw from a USB port (500 mA); the included Apple standalone power adapter can supply double that (1 A). The difference is even more dramatic for devices that have higher current draw to recharge (newer model iPads, for example).

Generally speaking, charging from your computer won't affect the performance of your computer at all as long as you aren't syncing or otherwise interacting with the device. USB can supply power without requiring data transmission (such is the case for wall adapters).