@kanewolf - thank you for your reply. I know that NVMe would have a better performance but that is not my point. What I'm having issues with are the visible speed drops from 385 to 75MBps. I have tested a lot of SDDs and whenever I checked them the speed was always constant - the line in HD Tune...
I just bought a brand new laptop (Asus Vivobook 17 Pro) with SSD M.2 256GB from Hynix. After I updated Windows 10 to the newest version I decided to run HD Tune benchmark on my disk. No apps / programs were running in the background. As you can see on the screenshot there are a lot of speed...