Slow travel only 1mm/s

You have LightBurn set to mm/min and the layer set to 20 mm/min.
The machine console displays speeds in mm/s, with a minimum value of 1 mm/s.
The actual speed is much lower, because 20 mm/min = 0.33 mm/s.
CO₂ lasers generally use mm/s. To avoid confusion, you should switch LightBurn to use & display those units: Edit → Settings → Display & Units will get you there.