In addition to what was described by fusion, there's something else to consider. In the above example, both drives were the exact same speed, you'd think there was no difference.
However, while your system is ripping data off the HD, it is at the same time writing data to temp files and swap file. If it's reading the data for your programs off the same drive as it's writing to, it has to alternate between one action and the other ... read then write .... read then write.
If they are separate drives, then the two drives can act independently. With samll programs, you'd never notice the difference.... with large programs and files, it's quite significant.