It's an estimate of rtt + dns resolution. The challenge in providing a good dns service is more about having nodes closer to the user than the dns resolution s

It's an estimate of rtt + dns resolution. The challenge in providing a good dns service is more about having nodes closer to the user than the dns resolution step itself because that in theory is almost constant for cached responses (it's usually in the microseconds) and when it's not cached the response time is really dependent on recursive querying of other dns servers. So the dns resolution can me estimated by measuring it from a data center manually and subtracting the rtt and use that as a constant. To estimate the rtt, you can do an xhr request and grab the connectStart and connectEnd using window.performance API keeping in mind it takes 3 rtt to do the handshake. Note that the request will fail for services that don't provide https support but that's ok because we just want to measure connect. The reason cloudflare has a better performance is most likely because they have better coverage and not because their servers are faster. Faster servers are a small factor in the final response time. For services that provide https support you can accurately measure the connect and response time of the XHR request using window.performance interface and subtract the dns resolution of the request. Also if you do the request twice, the second time is likely to have a dns and connect time = 0.