I finally got a consistent speed test result:
http://www.dslreports.com/speedtest/37313200.png

My eero router automatically tests daily, but I was never seeing above ~300Mbps. They are using Ookla Speedtest.net as far as I can tell. I saw the same on Speedtest directly. Using DSLReports I initially got similar performance.

Once I turned on HTTPS however I started to see the correct speeds. My only thought is that Spectrum is transparently caching HTTP requests. Anyone have any more information?

My modem channels:
https://gyazo.com/24b535cca7af0c07879f8dcf31524785