var newElement = document.createElement("div");

newElement.id = "testing"

for (i = 0; i < 1000; i++) {

  var newElementPart = document.createElement("div");

  newElementPart.classList.add("testClass");

  newElement.append(newElementPart);

}

document.getElementById("test").append(newElement);

html = "<div id='testing'>"

for (i = 0; i < 1000; i++) {

  html += "<div class='testClass'></div>";

}

html += "</div>";

document.getElementById("test").insertAdjacentHTML("beforeend", html);