There is nothing doubtful about it. A normal mechanical HDD is nowhere near the limit of max SATA II transfer speeds.
A mechanical HDD with a SATA III interface is the same speed as one with SATA II if anything else is the same.
The fact that the interface offers higher bandwidth does not change the speed of the HDD.
I used to have a setup of Raid 0 Raptors for my gaming drive, and with that I had stuttering when entering an area with a lot of people, though not to the extent that others have reported.
Dropped that setup in exchange for an SSD and I no longer suffer stuttering.
It is not that people all pop instantly when entering an area, but there is no FPS hit (the stuttering) as there used to be.