When RSS_FLOW_CONTROL or SDS_FLOW_CONTROL are set to a value other than -1 Informix will try to limit log activity on the primary server so that the secondary servers do not fall too far behind. There is no easy way to know or report on when this occurs.
For RS secondaries the only way to know if flow control is active is to look at the output of "onstat -g rss verbose".
For SD secondaries I see no documented method.
We have thousands (6,000) of users each with multiple sessions tending to our customers.
Our workaround has been to disable RSS_FLOW_CONTROL to avoid performance issues on our production host and have noticed fewer complaints of slowdowns since doing so.
A query was written to monitor if flow control was active but the sample rate would be to slow to always know when the issue was occurring.
The suggested corrective solution would be for the engine to clearly state when flow control is active.