Question about DX517 - 5 bay expansion unit

[Enquiries]

In the downloadable specifications for the DX517, in the notes it states "To have better performance, we recommend creating
storage volumes physically located within the same
NAS or Expansion Unit, rather than creating volumes
across your NAS and Expansion Unit"

I'm guessing this means that the overall disk i/o performance of the drives in the expansion unit are not the same as for the drives in the actual NAS unit. However, I couldn't find anything about the overall performance of the drives in the expansion unit.

I was wondering if you knew the reason for this restriction in the notes and what, if any, performance hit is there in the expansion unit. I'm thinking that this means that for the best performance, you are better off buying a NAS with the desired number of drives instead of adding an expansion unit to a smaller NAS. (of course, assuming memory, processing power, etc are all the same).

Curious if you had any knowledge or experience with this issue?

Thanks!