Sometimes, performing a QUERY SELECTION in a small current selection could be much slower then performing a sequential query. This is true if the current number of records for the table is very large.
You can detect this case with a function that calculates the ratio of selected records to all records, and perform a QUERY SELECTION BY FORMULA, which will force 4D to perform a sequential query.
You can also use the SET DATABASE PARAMETER command and the Seq Query Select Ratio constant (5) to specify a new value. In this case, you could still use the QUERY SELECTION command and 4D will switch automatically the kind of query for you.