We took note of the drive’s temperature during some of our benchmarking runs.
The only form of heatsink that the Lexar NM790 appears to have is a very thin foil built into the label covering the NAND and controller. We reckon it really does need some form of third-party heatsink or a motherboard cooling solution as it's not only a very fast drive, it does also gets very hot when pushed hard. We did our testing using the hefty built-in heatsink on our Gigabyte AORUS X670E Xtreme motherboard.
During our benchmarking runs, the hottest the drive got was 68° C during CrystalDiskMark 8 Sequential QD1-32 T1 Write test and the ATTO test, which is only 2°C away from the stated maximum operating temperature of 70°C, which is very close for comfort. But despite this, we didn't appear to see any throttle back during our tests. For the non-4 K tests the drive averaged 57°C while for the 4K-based tests, the average was 54.8°C.