Your Anvil benchmark test shows your SM951 is basically performing as it should. Your Asus board has the appropriate M.2 slot that allows an SM951 to perform at its best.
The 4K (single small file) read speed is lower than what this SSD is capable of, but that is probably caused by the CPU power saving options being enabled in the UEFI/BIOS. Drive IO does not load the CPU much at all, so the CPU cores stay at their idle speed and have transitioned into a higher C State (C6) and are barely operating. That adds latency which causes reduced IO speed for some types of IO.
Also, an OS drive will have slightly reduced benchmark results compared to a non-OS drive, since an OS drive is still serving the needs of the PC.
A 256GB AHCI SM951 should have a total Anvil score between 8,000 and 9,000, so yours is fine.
thanks, Should I disable power saving if it is on?