async-sequence-slow-fast.js

let timeTest = timeTest_fast; // timeTest_slow;

let startTime = Date.now();
timeTest().then(() => {
  let finishTime = Date.now();
  let timeTaken = finishTime - startTime;
  alert("Time taken in milliseconds: " + timeTaken);
});

async function timeTest_slow() {
  // ~9000
  await timeoutPromise(3000);
  await timeoutPromise(3000);
  await timeoutPromise(3000);
}

async function timeTest_fast() {
  // ~3000
  const timeoutPromise1 = timeoutPromise(3000);
  const timeoutPromise2 = timeoutPromise(3000);
  const timeoutPromise3 = timeoutPromise(3000);

  await timeoutPromise1;
  await timeoutPromise2;
  await timeoutPromise3;
}

function timeoutPromise(interval) {
  return new Promise((resolve, reject) => {
    setTimeout(function () {
      resolve("done");
    }, interval);
  });
}

// https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Asynchronous/Async_await