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Are you sure, that it moves with travel speed over the pants? To debug the generated gcode I always used this gcode visualizer https://ncviewer.com/ and I see on that area is everything ok with the speed and laser power. So I think your problem is the surface. I also had problems with such piece of wood. If its possible try another woodpiece with constant surface. Like I said above hardwood was the best for my grayscale pics.