NVMe drive tests at 1500 MB/s when it's the boot drive, but 3000 MB/s when not

If I boot off a regular SSD, my NVMe drive reads/writes at around 3000 MB/s. When I boot off the NVMe, speeds are only around 1500 MB/s.

Any hints as to why this might be happening?

Motherboard is ASUS ROG Strix B550-I Gaming and the NVMe drive is PCIe 4.0.