If you have an iPad charger, I would use that instead.
These Lithium battery's in the iPhone are of high quality, and should be able to charge AT LEAST at 5c the capacity without doing any damage to battery memory or life of the battery.
Meaning (for ease of explanation)..
Let's say the iPhone 6 has a 1amp battery (1000mah) it will take a 1 amp output charger to the charge that battery from 0 too 100 in one hour.
The C rating (and again I assume since these are high quality batteries in the iPhone... has to be and least a 5c rating, some Lipo and lith ion batts have upwards to 50c or more ratings) is how many times the rating of the battery can be safely charged by the output rating of the charger.
The C stands for charge rate. And EVERY rechargeable battery has its own Charge rate.
The higher the quality of battery - the less internal resistance the battery has, meaning it can be charged safely at a higher rate. And keep its voltage for the duration of the cycle (meaning it'll stay at 4 - 4.2 volts until the battery is at 10 percent or lower)
So if that 1000 mah battery, can be safely charged at 5c, it means a 5amp output charger will take that battery from 0 to 100 in 20 min. (Obviously slower charging will give the battery less strain)
So now let's take the IPhone 6s 1900mah (1.9amp battery) and charge it with the iPads 2amp charger. It will charge that 1.9 amp in that iPhone safely in less than an hour. (Just above a 1C rating - which is beyond safe).
And iPhone plus 2.9 amp battery. You can safely use the iPads charger at 2 amps (which is well below a 1c charge rate) and charge it in about 1 hour 30 mins.
Some batteries used in RC applications are well above 9000mah with a 80 plus C (charge rate) and can be charged with Pro grade chargers in less than 15 min. (Multiply at a time. With series and parallel charging. But that's a whole other thing).
So yes, if the iPhone 6 vanilla and plus come with the same charger as the iPhone 5s, it will take longer. But there are plenty of USB bricks with various amp output ratings you could get on the cheap to safely charge much much faster. If I knew the Exact C rating of the iPhones batteries I could tell you how fast you could charge these things safely.