Samsung Galaxy S6 4.6MB/s Seqential Write (Why so slow?)
I'm getting 45 MB/s sequential write when it should be 120 MB/s. Read speeds are roughly the same as on the review though.

In your case, I would try full factory reset, reflashing the stock ROM or even a custom ROM with a different kernel to see if the benchmark would generate different results. If you get roughly the same results after all, it's a hardware problem.