I stumbled across Arquillian the other day to do in-container tests for JBoss: http://community.jboss.org/docs/DOC-14376
I haven't had the opportunity yet to give it a full test drive but it looks pretty promising.
Spring also has excellent facilities for unit and integration testing: http://static.springsource.org/spring/docs/2.5.6/reference/testing.html
, which i use a lot myself.
The thing that impacts performance for our application the most is by far the database: some poorly written queries, joined views, slapping indexes on just about everything, bad caching, etc. really slows stuff down.