The more data you put on a HDD, the slower it gets. This is because it starts writing data at the edge of the disk and gradually moves inward as more data is written. 

Now to the physics part. When a disk or circle spins, the outer part of the disk spins at a higher velocity than the inner part of the disk. Pretty much it can read/write more data at the edge of disk than it can at the inner part.

But I have over 830 gb free on both of them

Ohhh, I misread your initial statement. Don't know then.