I had to try this for myself. There is no difference on my machine between an internal and external runtime as far as the Poser shader tree goes.

This image took 22'16" with the internal runtime, 22'12" with the external. The only other thing it could be is the sky... I have some AsileFX skies that I got with the "What's new in Vue 6" tutorials that had some messed up clouds. They would render slowly or not at all. This was just one of the preset skies that came with Vue. More experiments are in order. :)