I completely agree that the list of reasons "why RTT of IPvX is faster/slower then IPvY" is very long. I have already mentioned a few, you have mentioned a few additional.
RIPE Atlas has a number of ongoing measurements, including pings to Facebook, google, etc. from all probes. Looking at FB, the *average* RTT on IPv4 (using 13K+ probes) is 19.76ms, same on IPv6 (with 7K+ probes) is 17.83ms. I don't know who said "2ms advantage for IPv6" but they were spot on. In case you're interested in the raw data, or are curious to do the averaging for precisely the same probes: https://atlas.ripe.net/measurements/public?sort=-id&id__gt=1000000&tags=system-well-known-target&toggle=all&page_size=100&page=1&type=ping https://atlas.ripe.net/measurements/86227489/ https://atlas.ripe.net/measurements/86206980/ Robert