code: function runSubqueries (subQueries, callback, collection) {
          var i,
              responsesReceived = 0,
              length = subQueries.length,
              joinedSet = [];//The array where the results are going to get stuffed
  
          if (subQueries.length > 0) {
              for (i = 0; i < subQueries.length; i += 1) {
  
                  collection.aggregate(subQueries[i], function (err, results) {
                      joinedSet = joinedSet.concat(results);
                      responsesReceived += 1;
  
                      if (responsesReceived === length) {
                          callback(joinedSet);
                      }
                  });
              }
          } else {
              callback([]);
          }
  
          return joinedSet;
      }
docs: Run the sub queries individually, leveraging concurrency on the server for better performance.