I'm not familiar with this particular setup but, 5" per min is really slow. Increase your speed to max. If it is not cutting through the material at max feed rate, increase power until it does. If it is cutting through at max feed rate, reduce power if you are still getting the yellowing.
Also, check the focus of your diode to make sure it is at the smallest focus point possible. Focus on black paper & use a magnifying glass to help see the dot. You want it focused as close to the size of a pin prick as possible.
One of my lasers is only a 3 watt & I engrave wood at waay faster speeds.
Hope this helps.
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