I built a highway scene and the road lamps are all using emission lights with 1 sample. The lamps are also instanced along the highway. I noticed that using AI Light is doubling my render times. Any idea how this works? If the render times are getting longer, isn’t AI lights optimization not working?
If the only thing changing is AI lights "on" or "off", then yes, the render can take longer when on, although double seems a bit high. However, the idea is that with AI Lights enabled, you can significantly reduce other sampling settings and still get low noise levels, with overall faster results. You will probably see what looks like lower illumination levels with AI Lights enabled, also, since the rays from each light source are being more closely constrained. It's not a magic bullet; you'll need to tweak your scene and lights to take proper advantage of it.
I suggest that you try experimenting with extremely low Max Sampling settings (like Path Tracing, Max Samples: 8 or 16) and de-noising turned off in a very simple scene to get a feel for what's really happening. Go back and forth and use your A/B comparison as you fool around. Once you have a feel for it, turn up the Max Samples just to the point where your AI enabled lights look good. Then try it with them off. Now see how much higher you have to turn up your Max Samples to get the light noise under control again. The AI Enabled version will most likely render a lot faster with significantly lower Max Samples settings.