If you get slowness complaint from customer, you need to monitor database which sql is consuming a lots of resource.
You can read older Performance TOP SQL Scripts for Oracle database with following links.
Oracle DBA should monitor database everytime and if there are many sqls which is running long execution time then it should be reported to the developer and developer and dba should examine these sqls.
You can find TOP CPU SQL for last 24 hours with following script.
select * from ( select ss.sql_text, a.SQL_ID, sum(CPU_TIME_DELTA), sum(DISK_READS_DELTA), count(*) from DBA_HIST_SQLSTAT a, dba_hist_snapshot s,v$sql ss where s.snap_id = a.snap_id and a.sql_id=ss.sql_id and s.begin_interval_time > sysdate -1 group by ss.sql_text,a.SQL_ID order by sum(CPU_TIME_DELTA) desc) where rownum<20;