is it the difference in charging ports? micro usb vs apple proprietary charger? granted the samsung s2's battery capacity is a little larger (1650 mAh compared to iphone 4s 1432 mAh) but the difference in speed is huge. without any scientific testing and just from general observation from owning both, i'd say the iphone 4s probably charges about 5x faster. this is from my laptop usb to phone.