slowdown.js

export default function(awaitable, microseconds){
  return new Promise(async (resolve) => {
    const start = (+new Date);

    if ('function' === typeof awaitable) {
      await awaitable();
    } else {
      await awaitable;
    }

    const spent = (+new Date) - start;

    if (spent > microseconds) {
      resolve();
    } else {
      setTimeout(() => { resolve(); }, (microseconds - spent));
    }
  });
};