I'm curious as to why you would not be using KAIO and raw space, it is certainly easy to do in Informix?
You do not say how large your database is what have you tried running onstat -g iof to see if it's all spaces or just a few. i would look any any numbers that were larger than the rest. Map it back to you dbspaces and tables. Did you end up with all the heaviest used being together? Start moving data so that heaviest used is isolated.
crw-rw-rw- 1 bin bin 101 0x000000 Nov 6 2003 async
and in your informix profile put:
export KAIOON=1 # Boolean needed for HP
and if you have an issue with out of resources messages put this:
# range of 10 to 5000; Increase when KAIO: out of OS resources.