使用merge的三个j比不使用它慢

Question

如果我运行下面使用merge的代码，那么在不合并的情况下运行它，代码运行得更快而不合并。这与我阅读有关使用合并的所有内容相反。我做错了吗？

  var width = 800
  var height = 600
  var scene = new THREE.Scene();
  var camera = new THREE.PerspectiveCamera( 75, width / height, 0.1, 1000 );

  var renderer = new THREE.WebGLRenderer();
  renderer.setClearColor( 0xffffff );
  renderer.setPixelRatio( window.devicePixelRatio );
  renderer.setSize( width, height);
  document.getElementById(self.graphId).appendChild(renderer.domElement)
  var geometry = new THREE.CubeGeometry(10, 24.5, 12);
  var material = new THREE.MeshNormalMaterial();
  var merged = new THREE.Geometry();

  for ( var i = 0; i < 10000; i ++ ) {

    var mesh = new THREE.Mesh( geometry);
    mesh.position.x = Math.random() * 2000 - 1000;
    mesh.position.y = Math.random() * 2000 - 1000;
    mesh.position.z = Math.random() * 2000 - 1000;

    mesh.matrixAutoUpdate = false;
    mesh.updateMatrix();

    //scene.add(mesh);

    merged.merge(mesh.geometry, mesh.matrix);
  }

  var mergedMesh = new THREE.Mesh(merged, material);

  scene.add( mergedMesh);

  renderer.render( scene, camera );