No, it's not because the module is slower. It's because Apple only used 1 x 256GB chip instead of 2 x 128GB chips (which is what they did on the M1). With 2 chips, the system can do read/write on both chips at the same time, essentially doubling the performance. 1 chip means only 1x the performance, which is half the models with 2 chips. Apple uses the same chip for the 512GB model, but 2 of them, thus it has the "normal" performance expected.Most likely they opted for cheaper slower SSD module for that little bit extra profit margin.
Disappointing.
Basically Apple is too cheap to offer 512GB as base model, so instead of using 2 x 128GB chips for the 256GB models, they just decided to put one 256GB chip to cut cost.