There's a very good chance the 5400rpm drive is slowing it down.
Without further test, you won't know. You need to isolate each device and test them separately. You would need to find a HD performance test to see how well your HDs perform. Then you would need to find a test to stress your gigabit link. Since a gig link is good for over 100MB/sec, you typically need to use a ram drive to supply the data fast enough to actually attempt to saturate it.
I would think setting up a ram drive in OS X wouldn't be that hard since it's based on BSD, but I don't use/own any MACs.