Thanks Kevin for your reply.
Retailers where I live aren't really that friendly when it comes to returns except when there's a real obvious problem with an evidence. In addition, I have contacted WD by email and they nicely referred me to use Data Lifeguard Diagnostics for Windows, which doesn't show read and write speeds basically. I made the quick test and, as expected, it passed without showing any further details. I have had an experience few weeks ago with WD's Lifeguard Diagnostics, where I tested a 6-year-old (power on hrs) terribly slow driver but it passed both quick and extended test just because it doesn't have any bad sectors, I assume.
As for your question, it is relatively fast if compared to the old Green WD I have.
I made a internal transfer, from a partition to another one in the same HDD, just for the sake of comparison. The difference is not that huge and in my opinion doesn't worth the high price for the Black WD.
The new 2TB Black HERE.
The old 1TB Green HERE.