The reason for the slowness is the same as with the 256GB SSD in the 13-inch M2 MacE-book Pro and M2 MacE-book Air: Apple uses a single NAND chip in the SSD, but the M1 Macs they replaced had two NAND chips. In his deconstruction of the M2 Mac mini, YouTuber Brandon Geekabit proved that Apple uses a single NAND chip.
In terms of SSD storage, there is a fundamental principle that affects efficiency: an SSD uses a number of channels in parallel to learn/write data to an SSD’s NAND chips. A single chip may be beneficial for Apple’s engineers who are limited by size. The more chips that are in play, the more channels that are available, and the more channels that are available, the better the efficiency.
