1) Update statistics (The most common thing you can do first update you DB statistics if you are not doing this on regular basis.
2) Collect No work load statistics by using sqlplus command "
3) Collect workload statistics :
EXEC DBMS_STATS.GATHER_SYSTEM_STATS('INTERVAL', < minutes>);
4) Check alert log file for any warning errors common scenario:
a) ORA-01654: unable to extend
b) ORA-00600: internal error code, arguments:
To resolve these check your table space and increase them appropriately.
c) Check Point not complete ...Its a major issue that can effect your performance