I think we're seeing this across a lot of SSD's now too as higher density NAND becomes more available to source after the Silicon Shortage. First we saw the end of useful performance in 64GB SSDs, then 128GB after a while. While 256GB SSDs have been slower than 512GB SSDs for a while, it seems like the M1 MBP was immune to it, but now it's joined the rest fo the ranks where 512GB SSDs really are the minimum if you want unhindered performance.