Replacing the SuperDrive with a hard disk seems to be a Bad Idea at least in my 13" mid-2010 MBP: my 320GB WD Scorpio Blue took over a minute to copy 60MB. That is so 80s.
Interestingly, when I put the hard drive back to where it belongs, and put a SSD in the slot it vacated, the performance of the hard drive went back up to nominal.
Any one planning to upgrade to a dual SSD+HDD configuration should leave the HDD where it is and replace SuperDrive with the SSD.
Cheers,
Steve