NVME slowed inexplicably

I have a Linux box configured with the OS on an NVME drive “/” and separate physical drive contains “/home”.

I was having what I thought was diminished performance so, looked at write speeds using dd. I got 2GB/s on the NVME & ~107 MB/s on the /home drive. I figured it was time for the spinning platter to go and decided to upgrade it.

The new /home drive, an ssd now getting ~560 MB/s.
The untouched m.2 NVME drive speed went down from 2GB/s to ~760MB/s.

I can’t figure this and would love to know what is thought could be causing it.

The motherboard disables some SATA ports if m.2 installed, and I get the whole shared PCI-e bandwidth thing. However, I didn’t change any of that. After imaging from an old drive to an ssd I pugged the ssd in the slot where the old one was plugged.

In my mind, the m.2 shouldn’t have been affected at all.