"Performance" is a red herring. In a safety-critical system, what matters is the behaviour and the consistency. ThreadX provides timing guarantees which Linux can not, and all of the system threads are executed in strict priority order. It works extremely well, and the result is a system for which one can can understand the behaviour exactly, which is important for validating that it is functioning correctly. Simplicity equates to reliability. It doesn't matter if it's "slow" so long as it's consistently slow. If it meets the product requirements, then it's fine. And when you do the board design, you'll pick a part appropriate to the task at hand to meet the timing requirements.
Anyway, systems like ThreadX provide safety guarantees that Linux will never be able to. But the interface is not POSIX. And for dedicated applications that's OK. It's not a general-purpose OS, and that's OK too. There are good reasons not to use complex general-purpose kernels in safety-critical systems.
IEC 62304 and ISO 13485 are serious standards for serious applications, where faults can be life-critical. You wouldn't use Linux in this context. No matter how much we might like Linux, you wouldn't entrust your life to it, would you? Anyone who answered "yes" to that rhetorical question should not be trusted with writing safety-critical applications. Linux is too big and complex to fully understand and reason about, and as a result impossible to validate properly in good faith. You might use it in an ancillary system in a non-safety-critical context, but you wouldn't use it anywhere where safety really mattered. IEC 62304 is all about hazards and risks, and risk mitigation. You can't mitigate risks you can't fully reason about, and any given release of Linux has hundreds of silly bugs in it on top of very complex behaviours we can't fully understand either even if they are correct.
Neither QNX nor ThreadX are intended to be general purpose kernel. I haven’t looked into it for a long time but QNX performances used to not be very good. It’s small. It can boot fast. It gives you guaranty regarding time of return. Everything you want from a RTOS in a safety critical environment. It’s not very fast however which is why it never tried to move towards the general market.