Based on your screen capture, your Link Speed is 3 Gigabit and your Negotiated Link Speed is 1.5 Gigabit. Link Speed is indicative of the SATA interface of your machine - 3Gb = SATA II speed.
Negotiated Link Speed is the speed your machine and SSD have "negotiated", or settled on. This is the actual speed the two pieces of hardware are communicating. I'm not familiar with Mac Minis, but I do know that my 2009 Macbook Pro will "negotiate" 1.5Gb if I plug its SSD in to the optical drive bay. If I plug the SSD in to the normal HDD drive bay, it "negotiates" (or connects) at 3Gb.
To achieve the 500MB/s+ read & writes speeds that are specified by the 840 EVO, your hardware must connect at a link speed of 6Gb.
I hope this helps.