const perfCache = {start: {}, end: {}};
const perf = key => {
  if (key in perfCache.end) {
    return perfCache.end[key];
  }
  if (key in perfCache.start) {
    const [s, ns] = process.hrtime(perfCache.start[key]);
    Object.assign(perfCache.end, {[key]: (s + ns / 1e9)});
    return perfCache.end[key];
  }
  Object.assign(perfCache.start, {[key]: process.hrtime()});
  return perfCache.start[key];
};


perf('whatever');
setTimeout(() => console.log(perf('whatever')), 500);