JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>
<form></form>
<span id="log"></span>
<input type="button" id="fast" value="fast" />
<input type="button" id="slow" value="slow" />

JavaScript

function randomString(length) {
    var chars = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz'.split('');
    if (! length) {
        length = Math.floor(Math.random() * chars.length);
    }

    var str = '';
    for (var i = 0; i < length; i++) {
        str += chars[Math.floor(Math.random() * chars.length)];
    }
    return str;
}

function log(msg) {
    const elem = document.getElementById("log");
    elem.innerHTML += msg;
}

function addSelect(target) {
    log("Insert into " + target);
    var html = target + "<select>";
    for(var i = 0; i < 10000; i++) {
        html += "<option value='" + i + "'>" + randomString(30) + "</option>";
    }

    html += "</select>";
    const start = new Date();
    document.getElementsByTagName(target)[0].innerHTML = html;
    const end = new Date();
    log(" takes " + (end - start) + "ms<br>");
}

document.getElementById("fast").onclick= function() {addSelect('div')};
document.getElementById("slow").onclick= function() {addSelect('form')};