To draw or not to draw (rays)
The two most commonly used options to start a CPU based raytrace are "Trade and Render" and "Trace All Sources." The first option will draw the rays as they are traced through the system and the second option does not.
It's valuable to see the rays, but it's probably quite obvious that the act of drawing the rays might slow down the raytrace a little, therefore most users who are tracing thousands (millions) of rays select the non-drawing "Trace All Sources" option.
However the difference when choosing the "Trace and Render" option over "Trace All Sources" can be more dramatic than "slightly slowing down the raytrace" because when you ask FRED to draw the rays, this forces the raytrace to be single-threaded. In contrast, when using the non drawing "Trace All Sources" option, FRED can distribute the raytrace over up to 17 threads (cores), and FRED Optimum and FREDmpc can run on up to 63.
Therefore, assuming that the user has multiple cores at their disposal, the difference between "Trace and Render" - a single threaded raytrace that draws the rays, and "Trace All Sources" - a multi-threaded raytrace that doesn't, can be significant.