If you're following along with my posts on installing, initializing, adding space and creating databases and tables in Informix then you're ready to start doing some basic performance tuning. You can improve performance dramatically by making some simple changes to the ONCONFIG and bouncing the engine. The first and most important change to make is to increase Buffers, this is where Informix will be caching all of the data you are reading in from disk. More data cached means less I/O and less I/O has to mean better performance and since I/O will most likely be your current bottleneck, reducing I/O should result in the most performance gain.
How Informix Caches Data