I have a scandisk Z400S in my Inspiron 7559 and according to userbenchmark.com, I am getting terrible 4k read and write speeds.
My 4k read speed is about 8MB/s(average for all Z400S's benchmarked on userbenchmark.com is 24 MB/s), and my 4k write speed is about 25 MB/s(average is 84 MB/s).
The only possible cause might be that I seem to have some trouble installing the latest I/O driver from the dell website.
The installation says it completes successfully, but then Dell Update continues to suggest the driver as if it did not install correctly. Aside from this, I have install successfully all over recommend drivers from Dell, in the recommended order.