After reading Marcus Vance's article on NVMe SSD thermal throttling and DirectStorage sustained IOPS, I opened HWiNFO64 to check my 2TB PCIe 4.0 drive sitting in the M.2_2 slot right below my graphics card.
While Drive Temperature 1 (NAND Flash) read a totally fine 52°C, Drive Temperature 2 (ASIC Controller) was hitting 83°C after 40 minutes of Helldivers 2 and throttling my random 4K read speeds down by 70%! Moved the drive to the top CPU-wired M.2_1 slot under the motherboard aluminum heatsink (and verified the blue plastic film was peeled off the thermal pad), and Controller Sensor 2 dropped from 83°C down to 54°C under load.