Ars Technica reports the new M2 MacBook Pro’s 256GB SSD is only about half as fast as the M1's SSD.
Sustained disk read speeds run by Max Tech using the BlackMagic Disk Speed Test showed a drop from about 2,900MB/s in the M1 MacBook Pro to 1,446MB/s in the M2 MacBook Pro. Write speeds dropped from 2,215MB/s in the M1 Pro to 1,463MB/s in the M2 Pro, a smaller but still significant drop.
The culprit appears to be the NAND flash configuration. Both YouTubers took the bottom off of the new MacBook Pro and discovered that the 256GB versions use just one 256GB NAND flash chip, whereas the M1 MacBook Pro uses a pair of 128GB flash chips. On drives with more physical NAND chips, SSD controllers use a process called interleaving to read data from and write data to multiple physical chips at once. Use fewer chips, and you can limit your peak performance.
The Pro in Apple's product lines hasn't stood for professional in some time.