I wonder what toolkit you use to test your Mac device's writing and reading speed, and whether the result is objective and accurate. Commonly, the result varies along with the change of toolkit. You're highly suggested to test your Mac with other speed test software for a relatively accurate result, like iBoysoft DiskGeeker.
iBoysoft DiskGeeker can test the drive's read-write speed. It provides 2 modes for internal hard drives and 6 for external hard drives. Meanwhile, it detects more drive info and displays under the drive name, including the health state, temperature, etc. You can download this utility to test your Mac machine one more time!