I got myself an iPhone 6 last month. Because it has a larger battery than previous iPhones but the iPhone chargers have remained the same since the iPhone 3G days, I was a bit worried about how long it takes to charge the iPhone 6.
iPhones and other devices with a lithium ion or lithium polymer battery first charge to 80% fairly rapidly and then the charging slows down for the final 20%. At some point, And at some point, Apple started to declare the battery 100% charged even though it was still charging. The Battery Doctor app claims to show when the battery is really fully charged. (But beware of all the "optimizing" and advertising in the app. But hey, it's free.)
My old iPhone 5 with its 5.5 Wh (Watt hour) battery charged at a rate of about 1% per minute until the 80% point. On the charger, my new iPhone 6 with its 6.9 Wh battery charges at 14% per 15 minutes until it reaches 80%. But... connected to my late 2013 MacBook Pro, it charges at 11% per 10 minutes. So the iPhone 5 would reach 80% in 80 minutes, the iPhone 6 on the charger in 86 minutes and the iPhone 6 connected to the computer in 73 minutes. So that's almost a 15 minute difference. Then, it takes about another hour to for the battery to be completely full.
All USB ports and chargers provide 5 Volts. Under the original specifications, USB ports can deliver 500 milliamps (mA) or 2.5 Watts. iPhone chargers deliver 1000 mA or 5 W. As of about 2008/2009, Apple computers can deliver the same 1000 mA to Apple devices. USB 3 ports can deliver 900 mA. But it turns out that my one-year-old MacBook Pro will actually deliver no less than 2100 mA (10.5 W) to my iPhone 6, if we can trust the system (click About This Mac under the menu):
Coincidentally, this is the same amount of power that the iPad charger delivers. (Which you can use safely with the iPhone.) So actually it's a bit disappointing that the iPhone 6 only charges 18% faster with more than twice the power available. However, the iPhone 6+ has a 50% bigger battery than the iPhone 6, so for the iPhone 6+ the difference is very likely much larger.