for what you want, the start point is the onstat command, logged as informix.
* the main one for what you want: onstat -p
will give you the main 50 main indicators of your instance ( mainly the ones you are looking for )
for disk space, use onstat -d
for memory usage per session, use onstat -g ses
onstat is very simple to use and has many options thatwill give you almost all
the perf counters of your instance.
type onstat -- to have a brief help