Why is my new NVMe SSD slower than my old SATA HDD for large file transfers?

I just installed a brand new NVMe SSD, expecting a massive speed boost, especially for large file transfers. To my surprise, it's consistently performing slower than my old SATA HDD when moving multi-gigabyte files between partitions. I've updated drivers and checked BIOS settings, but nothing seems to help. What crucial configuration or software issue could I be overlooking that's causing this bottleneck?