I don't agree on the single PID explanation, if I run multiple conversions simultaneously I can see Windows create a PID for each conversion. Moreover, if I don't quit() I can see the Word instances stack.
I managed to install the same version of Word on both servers but still, the worst is that the environment with the extra time issue is the one with the most ressources. CPU and memory usage are very low.
I will get back on this issue tuesday, have a nice weekend