are you sure you did not get your 2W & 6W laser diode mixed up? The other one may have failed (2W?) because of driving it with too much amps. The fastest I can laser engrave photos with our 2W M140 LD is 80IPM with full power in black areas.
The Max amps for a M140 2W is 1.7a by the specs. As long as the amps are varying when engraving shades, you should be OK, if it is the 2W LD.
I fixed your YouTube link. The one to copy and paste is the browser link, then highlight it in your post and then select the YouTube button.