var StartTime = new Date().getTime();
BenchMarkTestFunction(); // render frame for example
var EndTime = new Date().getTime();
var ElapsedMilliseconds = EndTime - StartTime;

var AcceptableTime = 1000; // one second
var IsGoodPerformance = ElapsedMilliseconds < AcceptableTime; // some number being acceptable performace

if(!IsGoodPerformance) {
  alert("Sorry your browser is not good enough to run this site - go somewhere else");
}

您可以通过测试您的网站在不同的浏览器/设备，并查看它的执行方式和ElapsedMilliseconds的值。

You can determine what the AcceptableTime should be by testing your site on different browsers/devices and seeing how it performs and what the value for ElapsedMilliseconds was.