On mine with a single graphics card, the slot closest to the CPU, under the primary PEI-e slot runs on average 5c hotter than the other.
All m.2 storage cards will throttle performance due to heat, look up the thermal thresholds for each m.2 device and place the one that has higher tolerances in the hotter slot.
I never understood why mobo designers place these M.2's in what seems to be the hottest locations on the motherboards right under the GPU heatsink fins.
