even in the case of soft-realtime we don't say the system is real-time because it's fast. We say its realtime because majority of the time it meet its deadline.
Yes, at the same time I don't know if I'd characterize a soft-realtime system as slow. One way to meet deadlines is by being very efficient/fast, while keeping variance low (e.g. no stop the world gc).
That would indeed be a realtime system by common definitions as long as that latency is guaranteed. Audio and video are commonly considered soft-realtime domains for exactly that reason.
And phone … callthatju arousmpnnd is n’t realtime.
Technically not relevant but I know that a lot of early handhelds that repurposed RTOS for GUI felt slow, jumpy and unresponsive, perhaps because everything was polled? In that sense RTOS can be slow.
I think it felt that way because of bad design + slow hardware. You see the exact same thing by installing a modern windows version or some desktop like GNOME to low-end computers.