Ultra M.2 SSDs have read speeds ~2500MB/Sec.
- Roughly 5x the speed of a good SATA-III SSD
- Cost is roughly 50% higher than a good SATA-III SSD
Most folks don't need read speeds anywhere near 2500MB/Sec.
Folks that need this type of performance; composers doing huge orchestral mock-ups... where they're pulling 2000-4000 notes of disk-streaming polyphony from "multi-mic position" sample libraries.
A PCIe x4 SSD will also sustain ~2500MB/Sec on reads.
PCIe slot configuration (and how many PCIe lanes available to that slot) has a major impact on performance.
Cost of Ultra M.2 and PCIe SSDs is roughly the same.
A pair of good SATA-III SSDs in RAID will net ~1000MB/Sec on reads.
If you're running a demanding sample library that only allows a single disk location (ie: East West Symphony Orchestra), this is an effective/affordable solution.