I put together a lighting engine this week. It took a lot of coding before I could really test anything.
My first test effort I just rendered things as boxes to ensure that I even had the correct data.
Next I did some ray casting so I could see if I got that working.
Lastly I rendered those rays using a triangle fan.
I found that the CPU was being maxed out pretty bad, it turns out my first step (showing the boxes) was using the most. So I turned those off and things ran at a nice clip.