The SFF community doesn't like to talk about this but it's becoming an issue: NVME drives overheat and throttle. Guru3D has just put benchmarks to what we all hate to admit, and it's eye opening. In their tests, they saw the bare NVME drives cut their speed to one-sixth of the speed of a drive with a cooler. This is not a small difference and not a good sign for those of us running bare drives on the back of their ITX board (myself included).
Check out the Guru3D article by CLICKING HERE.
In the meantime, what can you do as an SFF user? Most ITX boards have some sort of heatsink for at least one of the NVME drives. Some even have fans. Put you're fastest drives in there. If you have to use a bare NVME drive, try not to use that drive for extended file transfers. Finally, try to make sure your case has adequate cooling. Even a small amount of air flow can help.