Until Informix Dynamic Server 10.0 the number of MSC (or Miscellaneous) Virtual Processors was limited to one. With IDS 10.0 multiple MSC VPs can be configured using the VPCLASS onconfig parameter (e.g. VPCLASS msc,num=2), or added dynamically using onmode -p 1 msc.
What does that mean and why would I want to use it?
The MSC VP is a type of AIO VP, executing operating system calls on behalf of other threads. While the AIO VP mainly takes care of filesystem I/O calls the MSC VP executes some network and authentication calls like getpwnam(), gethostbyname(), gethostbyaddr() etc. (It also maintains some statistics like VP CPU usage.) The MSC VP gets used at client connection time for the operating system part of user validation, and in most cases one is sufficient.
Get to the point
In some cases, for example when a connection pool creates a large number of client connections at once, and/or when a PAM (Pluggable Authentication Module) implementation causes the operating system part of user authentication to take longer, the MSC VP can either be very busy serving the new connections or waiting for the OS to respond. When this happens new client connections can take longer to service and users will see a longer delay when connecting for the first time. Situations like this might benefit from having an extra MSC VP. Running onstats like onstat -g iov, onstat -g glo and onstat -g stk <tid> can help show what the MSC VP is doing but keep in mind there are other possible causes when new connections experience delays (such as operating system and network problems) so some troubleshooting (and tech support assistance) would generally be required, but at least dynamically adding a VP is an easy test to make if you suspect this might help.