We upgraded from JBoss 4 (and JDK 5) to JBoss 5 (and JDK 6). The problem is that the start time has gone from 1.5 minutes (on JBoss 4) to more than 4 minutes.
Do you need the JMX console application? Pragmatic thing would be to un-deploy it from the server, you could still use the jconsole or jvisualvm for basically the same thing.