Well the thing with filling up a HDD to max is that the R/W head is moving towards the centre of the platter and thus the circumference of the region it's flying round gets smaller and smaller as the HDD continues to fill up. The head's flying over less sectors = less data flown over per second = lower speed.
Yes, that is how physics works, the more data you write to fill up the drive, then it has to slow down as it gets filled up.
He was just asking what is the max for a WD Purple, and that is basically the whole drive.
The purple is usually for surveillance, and that is one long stream of data, so, fragmentation is a non issue, there isn't lots of deleting going on here.
They even got a special mode for doing this kind of workload.