@AnkerSupport: I just tested my P20 with Anker charger with Power IQ1 and IQ2 tech. I didn't have voltage meter for USB C to USB C. But here is the initial result.
Anker chargers, with Power IQ1&2 port, can charge the P20 at the speed of 5.2V x 1.82A when the battery was around 66%. I tested with both Huawei cable (purple head) and Anker cable. There was no difference when using the Anker cable and Huawei cable to charge with Anker powerport (2&4). Anker cable could also work with Huawei charger but can only charge at the speed 5.2Vx1.82A.
Anyway, that was some results of a small test. Basically Anker charger can charge P20 at the speed of 10-12W - which is fine for most cases.