Tuning performance on Informix
Performance tuning is an elaborate process. To approach performance tuning, keep in mind that seemingly unrelated things could affect each other. When tuning for improved performance, make sure you document the changes and performance of sample runs so you can always go back if you need to back out something that has made a negative impact on performance.
One of the big problems with performance tuning is that there are so many subsystems that it's difficult to know that something needs tuning. Previous experience and experimentation is helpful for developing an intuition of what is operating optimally and what might need a little tweaking. Experiment more to see if your intuition predicts changes that improve performance on your server.