I’ve a MacBook Air 2015 with an Apple SSD SM0256G and also a MacBook Air 2018 with an Apple SSD AP0128 M. Both with the latest Catalina installed, the 2015 is outperforming the 2018 in write speed. Why does this happen?
Make sure to keep at least 20–25% of your SSD’s space free, as write performance can significantly drop when the disk is nearly full. If the issue is due to insufficient disk space, you can use iBoysoft DiskGeeker to clean up junk files and free up storage.
Your 2015 model is equipped with a 256GB SSD, while the 2018 model only has a 128GB SSD. SSDs rely on multiple NAND flash chips working in parallel to achieve faster write speeds. The smaller the capacity, the fewer chips there are, which leads to slower write performance. Therefore, you might consider upgrading your device or using an external high-speed SSD.