message order issue under load

We are using a simple route to process a one way message.

We have modified the behavior of jetty and camel to have only one thread.

Jetty configuration :
<httpj:engine-factory bus="cxf">
<httpj:engine port="0">
<httpj:threadingParameters minThreads="1" maxThreads="2" />
</httpj:engine>
</httpj:engine-factory>

When processing time is higher than the time between two requests, after a while, the emitter is blocked and the order is not kept.