It's not strictly true that the rt-patchset is meant to give faster response times. It provides bounded response times. You get a guarantee that some code can run within the time frame that you specify. That's not necessarily faster.

I was at a meeting with people that were doing signal processing in "embedded" environments. They said the patch slowed them down a lot. They got better performance and more determanism even, by pinning processes to cores using prior knowledge.

This would kill most use cases for QNX+POSIX.

You'll still need QNX if you want to run a full GUI Unix-like OS from a single floppy disk.

That hasn't been the case for several years now has it, since perhaps QNX v4? If not, you'll need a time machine as well.

http://toastytech.com/guis/qnxdemo.html

http://qnx.puslapiai.lt/qnxdemo/qnx_demo_disk.htm

Recent QNX has removed Photon and encourages the use of Qt instead which while can be made small, certainly does not fit in 1.44mb especially in 64bit

Yeah, the usual size for a minimal Qt with either QWidgets or Qt Quick / QML is about 15 MB.

Do people still have this requirement in 2020?

Usually such use cases require a certain level of certification and security that Linux doesn't offer.