This appear be a problem with SET ISOLATION and not lock release (COMMIT/ROLLBACK delay)...
You need to debug a situation step-by-step and monitor how your locks are created and when they are released to make sure all is running ok.
Need make sure you aren't put any SHARED lock in any row .
If your table are small, remove the PK and create just a CONSTRAINT if need , probably the Informix not using the PK .
To see if the table are in memory , use the command "onstat -P" .
If you work with version 7.31 , you can force the table in memory with SET TABLE xyz MEMORY_RESIDENT , if the version is newer this statement do not have any effect, Informix are smartest enough to keep the table in memory.