As a real-time OS it is known for deterministic response times. If it were exceptionally fast (and licenses cheap enough), you'd see hosts in the TOP500 using it.
I agree 100%. QNX is lurking in products you may use. It was the OS for the show control system that is used throughout entertainment where real-time is necessary for the safety of the devices it controls, which also have hardware safety at the lower level. I would drop into the QNX terminal for certain tasks. Unfortunately, you used to be able to download the show control software and play with it, but it has since been bought buy a company that sells it with the equipment they rent, so you need to buy trainging and it is behind their wall now. Not QNX, but the show control software that runs on QNX.
I'd like to see more information about that. I remember that Penguin Computing offered such some 15 years ago, but don't know where it was deployed or still is. Cray and IBM had also such a concept for their superclusers in the past, but are they still using such? The one HPC environment I worked on (a major car manufacturer in Europe) used plain RH Linux on all nodes as recently as three years ago.
The current #1 (Fugaku) uses IHK/McKernel as kernel for the actual payload. The previous #1 (IBM Summit) seems to use RH Linux though. Perhaps, since the most performance critical part is run by and within the GPGPU(s), the actual OS doesn't matter all that much (for performance -- it matters of course for programmer's comfort/efficiency).
There used to be a lot of "special microkernel on compute RPCing to Linux on I/O" on Crays and the like. Hard to say how prevalent it is now, and most annoyingly I can't recall the names. (Charon?)
Full disclosure: I maintain QNX toolchain.