Your speed is being limited in the firmware. Using g-code, there are values you can play with to increase your speed:
The default values for these three settings (defined by V1) are appropriate for using your machine as a CNC router, and take into account the much larger mass being moved around on an MPCNC compared to a 3D printer or a dedicated laser engraver.
Changes to these values can be saved using an M500, but then you would need to change them back when you use your machine as a CNC router. Personally, I change these values in my start g-code define in Lightburn, but I don’t use an M500. This means my machine reverts to the CNC-router-friendly values when the board is powered down.
Note when I made substantial increases in acceleration settings (M201 and M204), I began to get artifacts in my laser movement. See this topic. Some of this inaccuracy might not have translated into a problem for laser raster operations since the movement is in straight lines. And Ryan suggested additional values I might have played with to address the inaccuracy in this post.
If you do increase acceleration substantially, you might want to also play with overscanning values used in your Lightburn layer settings. In addition, you might want to run some tests to see if you need some scanning offset adjustment. Even a small amount of misalignment in the movement between the directions has a large impact on the quality of the resulting image.