I've noticed that after running the JBossWS-CXF testsuite against current AS8 master, which includes Undertow, the server CPU use is quite high. I've hence taken a thread dump: http://fpaste.org/IhH1/
In particular, attaching a profiler to the server, it can be seen that after the testsuite completes the only 2 running threads are going through org.xnio.conduits.AbstractStreamSourceConduit:transferTo / io.undertow.conduits.ChunckedStreamSourceConduit:transferTo , which are called millions of times even if the server is idle.