LPI (Lines per Inch) is the number of times per inch the head travels back and forth. At higher LPI values, the beam passes over areas it has already burned, so you don’t actually gain anything from a burning standpoint, the hole just gets deeper. (It’s useful for 3D engraving - not as much needed for image rendering. In my opinion.) And it generates a LOT of additional data for the buffer to have to deal with.
I explain why it’s a problem for large sized engraves in another tutorial here: