512GB PCIe x4 speed issues

Lounds

TS Maniac
Hi

I got a NVMe drive pretty affordable, when I first got it I tested it's speed and got very close to its rated performance. I then copied the partition from my old SATA SSD to the NVMe SSD. Since then I can't get the performance, roughtly getting half the numbers. Is it a Windows thing that's slowing it down, do I need to reinstall Windows 10?