"sorting" and "filtering" will produce two different results. The first organizes the records in the current found set into a specified order. The size of the found set and whether the field on which it sorts is stored will affect how fast a sort takes place. The second would, presumably reduce the number of records in your found set by omitting specified records. (this is done with "constrain found set".) If you constrain the found set, then sort the records, it should take place more quickly as you have reduced the number of records to be sorted.