I think the point you're missing is that "low performance" realtime applications exist and can often get by with more commodity hardware and careful software, e

I think the point you're missing is that "low performance" realtime applications exist and can often get by with more commodity hardware and careful software, eg. running a RTOS on x86 hardware. But that's usually not a viable option when your latency requirements are measured in clock cycles or nanoseconds rather than microseconds. For that, you need more specialized hardware that can do realtime and high performance.