Why doesn't an SSD perform well on a migrated OS X?

Question

macOS

I put the Kingston SSDNow A400 in place of the drive, rolled a bare Sierra onto it, which gave out ~ 280 MB / s for reading and writing in the Blackmagic disk speed test, then migrated my entire system to it from the HDD (using the migration assistant) and after that he the test gives at best 80 MB / s for writing and 120 MB / s for reading
. What could be the reason? How to overclock it to the same 280 at least?
I really don’t want to install the system again from scratch and again install all the software and transfer data

Answer 1

after that, the same test gives at best 80 MB / s for writing and 120 MB / s for reading

Apparently you measured the speed immediately after the migration, if so, then everything is fine, migration is a large amount of recording, there are no corny free places for recording, the speed degrades in such cases, as soon as the garbage collector works, the speed will return to normal.
Make sure TRIM is running and the speed should pick up after a while.

Answer 2

How exactly did the migration take place? If through copying the image of the entire disk, then most likely the problem is in the file system formatted with the settings for the HDD. And on the SSD, you need to format the FS in accordance with the manufacturer's recommendations. For example, the cluster size in the FS is important for SSDs. Fix by formatting the SSD with the correct settings and simply copying the system files to a new partition. In general, why put such a slow SSD? There are also faster ones. Is that a very old poppy with a SATA2 connector? In addition, it is recommended to put the SSD in place of the HDD, and in place of the drive - just the HDD.