I've used multiple QNX based user interfaces and the one thing they all had in common was their lagginess and unresponsiveness.

That's more of a function of your hardware resources and workload than the OS architecture you're running. E.g. a lightweight Linux desktop installed on present-day hardware will literally run with zero swap use - in fact, it will cache most disk access. (Unless you open something that hogs RAM, of course.) That may not be literally a soft-RT system, but from the user's POV it's just as snappy as one.