The time required to traverse each scan line inside the actual image will decrease with increasing speed, but most of the elapsed time is spent outside the image in the overscan areas on either side, where the laser head slows down to a stop, reverses direction, and speeds up to the engraving speed again.
So slowing down reduces the time spend outside the image and can actually reduce the elapsed time to engrave the whole thing.
Watch how far the head travels along each line at 100, 200, and 400 mm/s and you’ll see how it works.