This is mainly to do with LuxRender but I'm using Blender to control it, so sticking it here.
Within Blender I create a new default scene, switch the render to LuxRender and render the box & light. It takes about a second for the box to display and continues on for a few minutes.
If I then change the material of the box to Glass and re-render the scene is black for at least 1hr 30m (at which point I gave up). The log shows no issue and both CPU cores are on 100%. This is a low performance CPU (Intel(R) Core(TM) m5-6Y57 CPU @ 1.10GHz) without OpenCL/GPU rendering.
Is there any way to speed it up by reducing the accuracy of the refraction, amount of light bounces or similar?