The Anker 511 Nano Pro with the powerline 3 (usb c to Lightning) should charge your XS up to 18 watts max. This should get you from 0-50% in around 30 minutes but not to 100% until around 2 hours. This is because the charging curve slows significantly as you approach 100% to preserve battery heath.

You can test the actual charging rate using an inexpensive usb c voltmeter from Amazon to get a definite answer but it’s possible that you are just noticing the slower charging between 50-100%.