And if they really used that adapter, it's never going to be any faster. They'd have to use the lightning to usb-c cable (which still won't make a difference with how current iPhone electronics are implemented).
The 6s will benefit from up to a 12w charger by using the extra headroom to power the device if it's in use while charging. If the phone is idle with screen off, charge times will be the same between the 5w and 10-12w chargers. The + models can take some of that extra power and apply it directly to the battery. Lots of threads here about it.
I havent timed the entire charge but 3 hours sounds about right. Its the last 15-20% of the battery capacity that takes a long time to charge. Using a USB voltmeter, you can see that the phone will take 1.5a when charging under 80%, it drops to 1a or less after the 80% mark in order to save the battery health.
Also I noticed that when you're charging 95% and up, the phone is essentially only trickle charging at that point--again for battery health I'm assuming.