I have a 5.5w diode blue laser but having difficulty cutting through 3mm mdf. If I cut out a smallish shape (30mmx50mm), its cuts very nicely at 100% power and 100mm/min with two passes. As soon as I have bigger jobs to cut, it doesn’t cut through most sections. As if it gets tired or something knowing its going to cut a big job. I have tested various speeds, power, heights (laser to material with all in focus), air assistance and not, and nothing is working. I have also noted that my horizontal lines cut better than my vertical lines.
You said you have tried with and without air assist - what do you have for air assist on the machine? You would need to be injecting a good amount of air directly into the cut for it to make much difference. Higher wattage diode lasers use multiple emitters in a linear array, so the beam is a rectangle, not a dot, which would explain why it cuts better in one direction.
If your machine has a working Z axis, you can try stepping the Z down a bit with each pass to set the focus deeper, and that can help too.
I bought a Hailea 55L/min air pump with home made pipe and nozzle. The smaller pipe restricted the flow too much, so used a much thicker hose with plenty air coming through the nozzle. I tested the air assist with a small shape and thought this is making an amazing difference. I immediately tried the peacock and it didn’t make any difference in the end. I can try to inject the air flow into the cut a bit more to blow out more debris. Yes it does have Z travel and not making much of difference with regards to my problem. I have tried with and without z travel per pass. I wonder if I could have a power supply problem? Does the GRBL settings look in order? I have done a lot of testing and adjusting and this seems to work the best. The guy who built the machine, CNC router with the laser attachment, says I must look at the x and y acceleration ($120 &121) but this is cutting so slow I don’t think that’s the problem. I tried to adjust it up and down but wasn’t helping at all.