const fragment = document.createDocumentFragment()

​

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

  const innerDiv = document.createElement("div");

  innerDiv.className = "MyClass"

  fragment.appendChild(innerDiv);

}

​

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

html = ""

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

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

}

​

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