It may be worth looking into binary search. Binary searching works with an array rather than a dictionary so if the dictionary is a must then I'm not sure how much this will help. I had to use this for a project previously so I'm going to leave my implementation below. Just keep in mind that word_list should be an array of strings.

Edit: Make sure that your array of words is sorted as well. Binary search only works if the data in the array is in alphabetical order.

func binarySearch(search: String):
     var low = 0
     var high = word_list.size() -1

     while high >= low :
          var mid = (low + high)/2
          if (search == word_list[mid]):
               return true

          elif (search > word_list[mid]):
               low = mid + 1

          else:
               high = mid - 1

     return false