What is an SSD drive's negotiated link speed?
In this case:
The Toshiba TS256C is operating at its fastest factory designed speed.
Basically, you bought an Apple computer with an SATA III (6Gbps) interface and an SATA II (3Gbps) Hard Drive, and an SATA I (1.5Gbps) optical drive. If you were to buy a different model of SSD or ODD that is SATA III rated for speed you could take advantage of the 6 Gbps speeds on either interface.

Apple has been known by the spec tracking community to do this for various models since around the time they introduced SSD's. It's a silly thing to do for a company that only officially supports a couple of after market internal hardware upgrades, basically limited to Crucial branded memory, AFAIK. Why they cripple their machines this way is probably a business decision: it's cheaper and most consumers won't ever know/figure it out/be able to tell.