You will get a wider cut if your laser is not perfectly vertical. Cut a square and examine the edges in both the X & Y directions. If they are both vertical but of different widths, it could be the laser is not circular or, if cutting wood, it behaves differently according to the grain. I found 6mm balsa to be the worst wood to cut. 3mm balsa was also very charred.
I use a STM32F303K8 as the gcode interperter. 72MHz, 32bit ARM with hardware floating point math. I have not yet enabled the FP unit but the performance and available RAM are great. The board is $11US in a Nano 30-pin footprint. Only drawback is it is 3.3V which is not a problem for me.
Most of us go out of our way not to burn the household furnishings. We also really try not to aim the laser at anything shiny as the reflections are probably not eye friendly and are often somewhat random.
The tutorial you cited is for BenCutLaser by Ralph Freshour, our trusty system admin, not benbox. The number of passes will depend on speed, focus, glue, wood, laser beam vertical alignment, mechanical precision, and the ability to remove the burnt ash. I would guess at 6 to 8 passes at 100mm/min.