I took a quick look at the code and the thread pool in question appears to be related to WebLogic's multicast cluster implementation; furthermore, it looks like it can only safely have a single thread (so adding threads is not an option). There's step-by-step cluster troubleshooting guidance in the WebLogic documentation, or, alternatively, I'd bet that reconfiguring your cluster to use "unicast" instead of "multicast" mode would take care of the log message.
That said, I think it's hard to say whether the log message is related to the lowered performance. Thread dumps, JVM profiles, and wl statistic dumps could help with diagnosis.
Looks like problem with workmanager which you configured for application. The number of threads assigned to this work manager has reached the configured value in the maximum threads constraint. Please check your workmanager.
I recommend leaving this particular internal WM alone and avoiding any attempt to add threads - the code it invokes requires single threading. See my other post for more explanation, diagnosis advice, and a potential work around.