A teardown of the new Mac mini shared by YouTube channel Brandon Geekabit reveals that the 256GB model is equipped with only a single 256GB storage chip, while the same configuration with the M1 chip has two 128GB chips. This difference explains why the new model has a slower SSD, as multiple NAND chips allow for faster speeds.
We have confirmed with the Blackmagic Disk Speed Test benchmarking app that SSD read and write speeds for the new 256GB Mac mini are each around 1,500 MB/s, which is anywhere from 30% to 50% slower than read and write speeds for the equivalent previous-generation model, although benchmark results and real-world performance can vary.
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so those with M1 still might want to hold on to it. still waiting for more test benchmark results to see how much actual difference it makes in overall performance.
We have confirmed with the Blackmagic Disk Speed Test benchmarking app that SSD read and write speeds for the new 256GB Mac mini are each around 1,500 MB/s, which is anywhere from 30% to 50% slower than read and write speeds for the equivalent previous-generation model, although benchmark results and real-world performance can vary.
full article

New 256GB Mac Mini and 512GB MacBook Pro Have Slower SSD Speeds Than Previous Models
While the new Mac mini with the M2 chip has a lower $599 starting price, the base model with 256GB of storage has slower SSD read and write...

so those with M1 still might want to hold on to it. still waiting for more test benchmark results to see how much actual difference it makes in overall performance.