Tough to make specific suggestions when performance tuning (DB size, hardware, user load, etc...) but the majority of these articles & attachments find common ground on most onconfig settings while some will differ a bit so YMMV.

IBM Redbooks | Performance Tuning When Using Informix Dynamic Server With WebSphere

Oninit: references tab has a wealth of good info

PRS Technologies, Inc. and Peter Schmidt - a resource for Informix Database Administration