Not getting close to advertised download speeds

We pay for 2100 Mbps download speed, the Xfinity speed test shows between 800 and 900 Mbps download speeds over Ethernet, and actual practical use of my download speed never exceeds 400 Mbps, generally averaging between 200 and 300 most of the time.

I've already contacted support to try and lower the speed we pay for the more closely match what we're actually getting, but lowering our plan's speed *raises* our monthly payment, which is just astronomically stupid. In what universe would I want to pay extra for slower speed, or otherwise expect a certain speed and be given something completely different? Nothing about this is right, and if I had the ability to switch providers, I'd do so in a heartbeat.