My iphone Xs charges at same speed on 18 watt charger as 15 watt charger

Yes, that’s true. The 3 watt difference won’t result in a measurable change. And probably most of the 1:40 is going from 80% to 100%, because the charge rate slows to 5 watts above 80% to avoid overcharging.

Hello,

Hmm, your adapters and cables are both USB-C, correct. See this:

Fast charge your iPhone - Apple Support

Good luck.