JSFiddle - React, Tailwind, and code Playground

HTML

<div id="mydiv"></div>

JavaScript

var timer,
    num = 19,
    output = $('#mydiv');

function changeNum(){
    output.text(num--);
    //каждый раз задержка будет на 100ms больше
    if(num >= 0) setTimeout(changeNum,500+(19-num)*100);
}
setTimeout(changeNum,500);