Fundamental Difference Between Charging An iPhone At The Mains & Via USB Cable Linked To An iMac?
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).