FPrime is not a previewer. It is a renderer, and fast enough to compute and display your rendered image interactively. The workflow change this makes is best experienced firsthand, but even the demonstration videos on this site give a feeling of its power.
FPrime is particularly adept with complex scenes with many lights, transparency, reflections, high polygons, radiosity, and area lights. Like all 3D programs, FPrime's rendering speed is highly scene dependent. For simple low polygon scenes with just a few lights, both FPrime and LightWave render at roughly same speed. As scene complexity increases, (especially as transparency, reflection, area lights, and radiosity are added) FPrime's renderer tends to outstrip LightWave.
FPrime's interactive renderer is faster, but it also has an offline Render mode which is even faster. But even better, it is not just a simple rendered sequence saver like LightWave. FPrime's design is workflow oriented, with the powerful ability of progressive refinement. This simple but powerful idea allows FPrime to revisit rendered frames or stills and efficiently increase their quality iteratively. For example, you might create an initial very fast draft render of all of the frames of your project. As you're viewing that first low quality (but complete) sequence, FPrime is already working to repeatedly improve each frame. No work is wasted or quality lost. You never need to specify render quality in advance. Instead, you simply let FPrime's refinement "cook" for as much time as you want or have. You can also abort renders in the middle of frames and resume them later, which is particularly important with very slow or print-res renders.
A few examples may illustrate how progressive refinement changes workflow: