I work with Android APP's development, and my last work was with latency reduction.
I tried ICMP pings to a lot of destinations, TCP pings and also TCP stablished connection, and the spikes keeps happening.
After days trying to find the issue that was causing network spikes, I saw in tracert that the communication of ROG Phone 5s with router was not good, even close to router.
The most annoying is that a Moto G from 2015 and a Xiaomi Redmi Note using the same Wi-Fi has a very stable connection.
Also tried with 3 differente Routers.
This doesn't happen with RJ-45 USB-C adaptor, when connected directed by cable with router, so I guess that it can be a Wi-Fi driver problem, I don't know.
I saw that there's other people using Rog5 and Rog6 with the same issue there I had: https://zentalk.asus.com/t5/rog-phone-6/rog-phone-6-wifi-ping-issues/m-p/245809/thread-id/775/page/5