Those speeds are absolutely insane for such a small print. The laser will never ever reach that speed in such a short space. So your values are not a real test but just arbitrary. It looks like those letters are about 20-40 mm in width, so either you turn on a high overscan or use a maximum speed of 1000-3000 mm/min.
What you are seeing is some arbitrary output based on the acceleration curve that is different in both cases, but you never reach the actual values, as such, it’s just guessing.
In general: cutting speeds are 500-1500 mm/min, engraving speeds are 1000-10k maximum. Everything above is just marketing. Such a heavy laser head cannot move faster, only if you laser a 400 mm line, and then it reaches this speed in the middle part.
Additionally, if you use it for images later, the maximum speed is 7000 mm/min at grayscale, because the serial interface is not faster.