Oracle has improved the cost-based Oracle optimizer in 9.0.5 and again in 10g, so one of the most common causes of bad performance right after moving to 10g are related to environmental parameter settings (init.ora parms) and your optimizer statistics. Properly configured, Oracle 10g should always faster than earlier releases, both for PL/SQL and SQL, so it is likely that any slow performance after an Oracle 10g upgrade is due to initialization parameter settings or incomplete CBO statistics.
Remember, Oracle 10g is the world's most flexible and complex database, and upgrading to Oracle 10g is very tricky. Prior to putting your Oracle 10g upgrade into production, it's a best practice to thoroughly test the migration with a real-world workload. As an alternative, many shops obtain an independent Oracle health check to identify their sub-optimal configuration settings. Experts who frequently perform 10g upgrades know exactly where to look, and they can save you weeks of frustration.
To see the secrets, click below to see the most common causes of poor performance after a 10g upgrade:
http://oracle-tips.c.topica.com/maal8FjabJATTbLGJrib/
Also, see these notes on justifying an Oracle10g migration:
http://oracle-tips.c.topica.com/maal8FjabJATUbLGJrib/
Important 10g upgrade gotchas revealed
