The charging controller is configured to feed the battery with at most 1.5A.
USB-C controller is configured to negotiate at most 5V @ 3A (15W), 9V @ 2A (18W) or 12V @ 2A (24W). To make it charge at full speed, this needs to cover both charging current and the power taken by the phone itself.
In my experience, 10W is usually enough to let the phone charge at full speed.
BTW. Currently the kernel drivers are missing the necessary plumbing needed for USB-PD negotiated currents to be propagated to charging controller, so the input current is always limited to 500mA. This is why the phone currently charges faster at higher voltage (and sometimes doesn’t charge at all when in use while attached to a 5V charger) - the maximum current stays constant. Once this is fixed (the code already exists, but isn’t merged yet) and once BC1.2 is implemented correctly, it should be able to automatically fast-charge with 5V chargers as well.