If your laser resolution is set to .1 mm then you’ll possibly be processing 10 g1 move commands AND 10 laser power commands for every mm of travel which adds up to 20000 commands per minute at 1000mm/min feedrate. Communication must be fast and uninterrupted for this to work. I would suggest getting the sdcard working to troubleshoot further. I use marlin and found that my laser stuttered a bit after upgrading to 12864 LCD even when running from sd. I noticed my stuttering was occurring once per second, same rate as display refreshes. I studied firmware and found that the u8g library is quite the resource hog so I disabled screen refresh during laser operations to correct the problem. Unfortunately we’re really pushing the limits of what the firmware is capable of. You may consider grbl for laser engraving since its much purer and unburdened by display updates , temperature monitoring, etc…I’m in the process of switching back to mach3 myself. I enjoy coding but I’m spending more time rewriting firmware instead of getting my mpcnc dirty lately so its time to leave Marlin behind.