This is a very common warning, and it essentially means you may have some timing constraint issues in your design. In order to see these constraint issues, run "report timing -lint".
FYI, for any message code in RC, you can run "man" to see more information about the message.
rc:/> man TIM-11
Entry : TIM-11
Severity : Warning
Verbosity : Message is visible at any 'information_level' above '1'.
Description : Possible timing problems have been detected in this design.
Help : Use 'report timing -lint' for more information.