Not a bug. Yes, slow.
LightWorks as implemented by Archicad does not produce the highest quality renders since it was never based in the most current LightWorks capabilities and has not been updated in over five years. You will not be able to favourably compare Archicad output to Artlantis or any other rendering application.
1: More Quality: set Method to "Final", or "Best". Establish an "Undersun" to fill the soffits with light. Add colour to light. PhotoRendering Settings: Realistic Sun.. More quality requires fine craft: Buy my book.
2: More speed: Minimize polygons. Reduce the number of lights in the scene. Don't use refractive glass, especially overlapping refractive glass. Smaller renderings!!!
I recommend obtaining the student Artlantis. I have a book for that, too.