My dated Z800 enterprise server has an onboard SATA II controller limitation, along with other quirks, so to make full use of modern SSDs I required a work-around.
Not that a SATAII limit is to be scoffed at if you can saturate that with a good SSD, I'm using a Samsung Evo 850 (SATAIII), with a SATAII limit you'd see around 260 MB/s or so read and write, which'd be fine.
To make better use of a SATAIII SSD though; as a work-around you can connect them through a PCIe lane, which for me got this;
It might not be up there with latest M.2 speeds, but bear in mind; this is a 2009 server.
I used a PCIe to 2.5" SATA card; the Sonnet Tempo, it's aimed at Macs but works in this PC, there are others but I've not personally tested them.