Samsung 850 Pro — incredibly slow write in my build

Hi!

I have a build with i7-7700K, Asus Z170 MB (don't remember the model) and Samsung 850 Pro 256 connected to 6 Gigabit SATA port running Catalina 10.15.3.

I've noticed that the write speed is very slow — about 80 MB/s. Read is OK, ~500 MB/s. I've tested with dd and BlackMagic benchmark with pretty similar results.

That's the system report:

https://preview.redd.it/08wam86ok8l41.png?width=3992&format=png&auto=webp&s=5a7726adbd21aaf268a681298c6a693a126847b2

What's wrong with my configuration? Write should be about 5x faster.