Toms article explains it but for those who want details, the SSD controllers reserves some amount of NAND cells for fast writes, what they call SLC cache. What that means is, instead of writing 3 bits per cell (in TLC), they only write one bit per cell on those NAND cells. They do that because writing one bit is much faster than writing 3.

So the controller maintains some "free" cells for fast write. In the BLU version, the 980 Pro controller that's used here has a different strategy which keeps more cells around (110ish GB vs 40 GB) for fast writes. As you fill up your disk, the controller will try to maintain this. You will see a drop when you go above that because the controller will start consolidating the data (the controller will also move data in it's idle time, consolidating the SLC writes to TLC writes).

This is why you do those specific tests as a reviewer, to analyse how the controller works, but it's not a real life scenario. A much more interesting test which we used to do back in the day was measuring performance when your disk is filled up (say 80, 90%). This is usually where you see those SLC cache strategy no longer working (depending on how good the controller is, some just don't consolidate data and you end up having poor performances).

Anyway, it's a pretty irrelevant change for 99% of users and the SSD will be faster on all other scenarios.

Considering they did a hardware revision, updated spec sheets and new packaging, and changed to a better controller (from the 980 Pro), this is nothing like what WD did.