Ah, then the answer to your original question is that the speed is not the issue. The 970 is an NVMe drive while that adapter is intended to be SATA to SATA.
From the page you linked: "Note: It doesn’t support M.2 NVMe PCIe SSDs "
You'll need to use a SATA m.2 drive or a 2.5" SATA SSD and skip the adapter.
Z.
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