Tried on several devices - it consistently under-reads. I'm on 500Mbps broadband. Ookla and fast.com show 350Mbps via WiFi. Cloudflare only 274Mbps. Similar results on Ethernet. Over 400Mbps on a proper speed test. Can't hit 300 on this. I assume their downstream is capped?
I wonder if it could be that various handoffs with cloudflare's network are more busy, but we don't really notice because we're not doing speed tests on it. :)