docs: function isSlowerThan(amount) {
    // always logging
    var diff = process.hrtime(this._start)
      , micro = Math.round(((diff[0] * 1e9) + diff[1]) / 1e3);
    if(amount === 0) return micro;
    if(micro > amount) {
      return micro;
    }
    return -1;
  }