What is a "Net Resolving" test?

Question

I'm developing a solution that can detect a user's use of a proxy/VPN. As an example, I'm looking at the solutions at https://bot.incolumitas.com/proxy_detect.html On this page, one of the tests is "Net Resolving". Here are the readings of this test when using a proxy and when connecting directly without a proxy. Does anyone know what this test is and what it measures?

It's a not proxy

{
  "is_proxy": false,
  "info": {
    "dnsResolving": {
      "res": 0,
      "perf": 57.6
    },
    "canLoadScriptFromUncommonPort": {
      "res": 1,
      "perf": 596.6
    }
  }
}

It's a proxy

{
  "is_proxy": true,
  "info": {
    "canLoadScriptFromUncommonPort": {
      "res": 1,
      "perf": 1347.1
    },
    "dnsResolving": {
      "res": -1,
      "perf": 3509.3
    }
  }
}