Speed tip #1: Don't use IBL with enclosed spaces
IBL light in Lux is an “outside” light. It will be always outside. It's basically the light of the the sky. If you have an enclosed space then you are obstructing the light and the render will go like molasses. The only reason why you actually see something if because many environments are one polygon thin and so light “spills” through. With the Autolinear tonemapper Lux cranks up the aperture to insane level so that you can actually see something, but the scene will be incredibly grainy and it will not resolve.
Speed tip #2: Sun and IBL - not a great combination
If you want to use the sun use the sun, don't mix it with IBL. The sun will overpower the IBL and you will not see the benefits of using the latter. It will slow your render down though, because you are computing some pretty demanding lights. If you want IBL then get a nice HDR image (see hdrlabs.com) and use that for your light source. If you need to add a sun source use a meshlight, it's already calibrated for daylight and it will fit perfectly. Use a dis and turn it into a mesh light, crank up the wattage, place in the sky and you're home.