The Google Pixel 3 XL phone supports USB Power Delivery 2.0 technology with its own shipped Google 18W USB-C Power Adapter for 18 watts based charging. After connecting the phone to the charger it took about 82 minutes to fill up a fully drained battery on the phone as observed below.
At initial plugin, the Google Pixel 3 XL began pulling current while at 5 volts starting voltage. As the voltage increased to a 8.8 volts rail, the phone pulled higher current to achieve around 17.6 watts (8.8 volts @ 2 amps) charging with the 18W charger. After the first 10 minutes, the current kept on alternating between 1.5-2 amps but when reaching 40 minutes it decreased slightly to about 1.4 amps for the phone to charge at around 12.3 watts (8.8 volts @ 1.4 amps). From that point onwards the current began to progressively step down to about .4 amps along with a slight increase of voltage to 9 volts in the 2nd half of charging. The current then further reduced down to below .4 amps as the battery almost reached full charge. Click here to download the trace captured by the GRL-A1 power analyzer to view more details of the entire charging cycle.
The Google Pixel 3 XL expectedly works fine over USB Power Delivery to get close to max 18 watts charging with its own supplied 18W charger.