Actually, the Unreal render isn't running in real time as you build scenes. It needs to be compiled first, then you move around the result:

https://www.youtube.com/watch?v=g7CTPS-5JsE

This makes sense, otherwise you would need a powerful computer and several video cards to run it.

I suspect however, a lot of work would be need to be done in poser to support the subset of rendering, just like DS basically ripped out their view port functionality for the IPR, the environment tab, the render settings tab, and the surfaces tab to allow multiple renderers and their different settings. Choosing a renderer gave you different sets of render settings, applying the iray shader gave you a different set of materials settings, shader mixer added in iray blocks, etc.