Does Spark regularly oversubscribe their BNGs?

I am doing a rather large download and was hoping to get more than 50 Mbit/s on my 100/20 service. Mind you 50 Mbit/s isn't all that bad from the US.

However I noticed that the Spark BNG has an average RTT of 8s, at 11am!! This isn't even the busy hour of the day.