M.2 Slot under RGB cover use PCH PCIE 3.0 lane not CPU lane (According to manual). As far as I know if you don't enable raid array volumn on your system.
The performance gap between single SSD on DIMM.2 and PCH M.2 on AHCI mode were little bit.
W11 25H2 27758.1000 Core i9 7980XE 02007108 MCE ME 11.12.96.2535 R6E Modified BIOS 4001 SAMSUNG OG9 FW 1019.0 SSD 970 EVO PLUS 1 TB x 3 NVIDIA RTX 4090 GAME READY 566.36 64GB GSKILL DDR4 3200MHz JBL 9.1 Sound Bar DTS-X