What would be the first thing you would do if an end user complains that performance is poor?

I always used to come up with this question on forums and well as from the Application support, and a time with my first interview in India.
As for the answer to the question, one should touch upon all these points:
- Check alert log, for any errors and UN even entries
- Check for archive log destination/file system full
- Check for database locks # Click here for a notes on locks
- Check server performance (CPU/IO)
- Check network performance
- Check if statistics is up to date
- If new SQL, then tune SQL (maybe take trace)
- Check if any parameters have been modified recently
- Check if any new updates as been done from the Application team.

And few more from Oracle Online Documentation as well
1) Identifying and Gathering Data on Resource-Intensive SQL # Oracle Doc
2) Oracle9i Database Performance Tuning Guide and Reference # Oracle Doc
I recommend reading the documents from Oracle, which give clear understanding of all the above. Please add lines if you I miss something which is useful.