"The app was rewritten in Java 1.6 and deployed on WebLogic 11g and the app has become extremely slow."
You have copied to code, or really rewritten it? What kind of technologies are you using, for example, EJBs, JSF etcetera.
Also what resources is the application using, such as, for example, database, messaging...?
Could the JVM is working overtime, i.e., that it is constantly collecting garbage instead of running your application.
Some tuning tips for WebLogic are provided here: http://docs.oracle.com/cd/E21764_01/web.1111/e13814/topten.htm#i1142952