Its the SSD cache it has, it stores the most frequently accessed files in there so some bloodborne files would get moved in to it for example with it having to loading the same place after each death.
Also I think the bigger the drive the faster it is (1TB 5400RPM will be faster than 500GB 5400RPM drive).
I am going to pick up the SSHD I think. $77 is cheap.
Do you have a link to the best directions to follow with doing the swap?