What JDK and version are you using? What version of JBoss AS?
You're most likely experiencing PermGen issue if you're using the Sun HotSpot JVM (this is not a problem when using JRockit). What are the VM args you're issuing to the JVM when starting JBoss (see run.bat in Windows or run.sh/run.conf in Linux)?
Try something like this:
rem JVM memory allocation pool parameters. Modify as appropriate. set JAVA_OPTS=%JAVA_OPTS% -Xms512m -Xmx512m -XX:MaxPermSize=256m
or see SiA book by DAllen for more info on this. You can use jconsole (see %JDK_HOME%/bin) to monitor the permgen space usage in a dynamic graph. This usually increases every time you hot deploy in JBoss.
JBoss 6 is a Java EE 6 platform implementation and features the web profile among many new features like JPA2, JSF2, CDI, Bean Validation, etc. As far as performance and backwards-compatibility, that remains to be seen/proven.