There are variety of reasons of a performance drop, is your PCIe port NVMe or SATA ? If you connect a SATA device to PCIe port with an adaptor, there’s no gurantee of the performance you will be getting, because the product is not designed to work that way, and I have no idea what the adaptor does during transfering signal, so my suggestion would be find a SATAIII port and re-test about speed.
I understand the drive works under your current driver, but drivers may cause the drive is not up to full speed too, that’s the reason I want you to update the drivers
