It's the Latency, FCC
circleid.com
circleid.com
Latency is tricky, though. ISP controlled latency is only manageable on the ISP network which could be huge (like someone riding ATT across the country from start to finish) or very small. So that would mean basic QoS and would only be relevant if the network gear was overloaded and had to pick certain packets to prioritize. Plus, with BGP a preferred route might get cut and a much higher latency route takes its place.
It's much easier to regulate and police transfer speeds while latency tests would only be relevant from CPE to the test server. We run our own speedtest server and customers in town don't even leave our network to hit it. Customers a couple of towns north have a lot more fiber to go through to get to that same server, so either the allowed latency unreliability would be high or ISPs would just put speedtest servers at every POP to game it.