From LightWiki

The reason it doesn't make sense to multi-thread a simple scene is the overhead of both processors talking to each other. Let's say, purely hypothetically, that the overhead is 35 seconds, and that because you are using two processors, the render times are halved (hypothetically - to make the maths easier!). If you take a one minute render, that should be thirty seconds, but uh-oh, there's that overhead making the final render times one minute and five seconds - slower than if you'd just rendered on a single processor. Instead, take a frame that takes an hour. With two processors you can render it in half an hour, then add the overhead of 35 seconds...

Additionally, with the advent of LightWave v9 there are two things to bear in mind. One is that your machine is automatically set to use however many threads your processor(s) support(s) with the Perspective camera, so you may want to turn off multi-threading on simpler scenes, and the second is that is a lot more pro-active when it comes to multi-threading. Previously, if you set a frame to use 8 threads as soon as one was complete you lost that thread, resulting in a gradual slowing down of rendering. With the Perspective camera, threads get reused, so as soon as one is finished, the frame gets divided up again to provide as many new threads as you have set.