Bad performance using SOAPProxy on SOA-P 5

Issue

We are load-testing an ESB service that contains a SOAPProxy action, and are seeing bad performance results. If we hit the webservice that is targeted by the SOAPProxy directly, average response time is ~300ms, but going through the ESB the performance degrades, starting with about ~400ms, but then going up to ~7000ms at the end of the test, averaging at ~3000ms.

This is the ESB service:

  <service category="Proxy_Routed" name="Proxy_Hello"
    description="Routed WebService Proxy: Hello" invmScope="GLOBAL">
    <property name="maxThreads" value="100"/>
    <listeners>
      <http-gateway name="Proxy_Hello"/>
    </listeners>
    <actions>
      <action name="proxy_hello_proxy"
        class="org.jboss.soa.esb.actions.soap.proxy.SOAPProxy">
        <property name="wsdl" value="internal://jboss.ws:context=Quickstart_webservice_proxy_routed_ws,endpoint=HelloWS"/>
      </action>
    </actions>
  </service>

Note: we have already set the maxThreads attribute on the invmScope'd service, which already improved performance, but the issue now seems to be related to the usage of SOAPProxy.