Async waterfall slow

I am trying to replicate http://pin1yin1.com/ (https://github.com/Pin1yin1/pin1yin1) which takes a string of chinese characters, groups into bands of 5 characters and tries to looks up the value in a database. If no matches are found it removes the last character and searches the 4 character string, until it finds a match.

For example 大猫头鹰 (big owl) will do the following:

1. Search 大猫头鹰 - no results
2. Search 大猫头 - no results
3. Search 大猫 - no results
4. Search 大 - result big
5. Search 猫头鹰 - result owl

I have this successfully working using Async waterfalls - however my solution is quite a bit slower than pin1pin1. Pin1pin1 is almost instantaneous result - even for strings with 50+ characters, mine returns in 15 seconds for 50+ characters.

Have I misunderstood pinpins algorithm or is my approach inefficient?

var returnArray = [];
var charArray = str.split(''); 
var i = 0; 
var j = 5;
ar searchWord = str.substring(i, j);

 async.whilst(function(){return i <= charArray.length - 1},

    function(sentenceCallback){

      async.waterfall([
          function(callback) {

              dictionary.where('s', searchWord).fetchAll().then(function(words) {
                  callback(null, words);
              }).catch(function(err) {
                  resolve(null, err);
              });
          },
      ], function (err, result) {


          if (result.length > 0) {

            var answers = [];
            for (var item in result.models) {
                 var obj = {
                    s: result.models[item].attributes.s,
                    p: result.models[item].attributes.p, 
                    d: result.models[item].attributes.d
                 }
                answers.push(obj)
             }

             var obj = {
                r: answers
             }

             returnArray.push(obj);
             i = i + searchWord.length;
             j = 5+i;
             if (j > charArray.length) {
                j = charArray.length;
             }

             searchWord = str.substring(i, j);

          } else {

            // is null
            j--;
            if (j > i) {
               searchWord = str.substring(i, j);
            }
          }
          sentenceCallback();
      });

  }, function(){
      resolve(returnArray, null); 
  });

via Starchand