Next, speeds above 3000 are already critical with the mechanics of the S9. If you want to have high-quality images, use speeds around 2000-3000mm/min, not faster. You can go faster, but you will need to carefully adjust the mechanics and the settings.
The material will have another influence. Wood sometimes has an internal structure that gets exaggerated by the laser process.
Finally, I’d recommend to use a scan angle of 45°, which will decrease the influence of a single axis to the result.