Heavy performance drop on Progress query, looks like based on string character inside table

I'm launching following query on a Rel 11.6 Progress database:

`
FIND FIRST Table1 WHERE Table1.Field1 = 1
                    AND Table1.Field2 >= 2
                    AND TRUE
                    AND
  CAN-FIND(Table2 WHERE Table2.Field1 = Table1.Field3    /* join */
                    AND Table2.Field2 = 0
                    AND Table2.Field3 = "WHATEVER").

` On both tables, the necessary indexes are created and rebuilt. The problem is the following: when the criterion Table2.Field3 = "WHATEVER" is present (although that field is present in an index), the query takes entire minutes to run, otherwise the query is done very rapidly (in a matter of seconds). What can explain such behaviour in a Progress 11.6 database and are there any profiling possibilities for monitoring what's happening? (I'm working with the appBuilder/procedure editor "IDE") Thanks in advance