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.
W11CANARY 26090.1 Core i9 7980XE 02007006 MCE ME 11.12.95.2499 R6E OFFICIAL BIOS 3801 SAMSUNG OG9 FW 1019.0 SSD 970 EVO PLUS 1 TB x 3 NVIDIA RTX 4090 GAME READY 551.86 64GB GSKILL DDR4 3200MHz JBL 9.1 Sound Bar DTS-X