How to tell if you have a MySQL performance problem

The top three things for mysql performance:

1) run 64bit OS on 64bit hardware with 64bit version of mysql (newer versions of mysql do better with large amounts of RAM if you aren't using 4.1+ you may not be able to use enough RAM).
2) make sure your indexes fit in RAM
3) Make sure you log slow queries and check the logs for problems. This is your number

Number 1 is easy.

Number 2 is a little harder. You can eyeball this if you are running ISAM databases by summing up all your IDX files and then seeing if MySQL is using more RAM than that.

But a really good way is to run a report: http://hackmysql.com/mysqlreport It has really good documentation and can make suggestions. You can use it to tune other options.

Number 3 requires a change in logging. You can also change the threshold- what is slow for you may not be for someone else. The problem with logs is you have to look at them, understand them and then act on them. Get going.