The base model of Apple's newest Mac mini and the 2023 MacBook Pro have has significantly slower SSD read and write speeds because of engineering choices, compared to that of the previous generation models.
A teardown by Brandon Geekbit has uncovered the reason on the Mac mini — the 256GB Mac mini comes with a single 256GB storage chip. Last generation's M1 Mac mini came equipped with two 128GB flash chips in parallel, allowing for faster speeds.
As it turns out, the 512GB M2 Mac mini also features a single NAND chip, meaning that it would still have slower read/write speeds than a model with two 256GB chips. However, as there was no 512GB Mac mini with an M1 Pro chip, there's no comparison to be made.
The situation is similar on the 2023 MacBook Pro as well, as discovered by 9to5 Mac. The lower storage capacity models have fewer Flash chips at a higher density than the previous model. This cuts down speed versus what it would be, versus what you get from parallelization of multiple flash chips.