A lot of those mcus don't need to run particularly complex programs. Many of them don't do anything more complicated than reading the status of a pin and using that to activate another pin. 75MHz with a Rust program is more than enough.

For that matter, when Python was first released, the Intel Pentium wasn't even out yet. People made it work. Micropython could handle the above just fine provided the polling rate didn't need to be that high.

It'll all depend on the application, of course, but a huge number of mcu programs could easily be Rust. The performance hit is not that big. The code size on lower end mcus is also going to be naturally limited by their flash space, so even the compiler speed isn't going to be a big hit.

One thing is that the last time I tried using Rust on an esp32, the toolchain was still in its infancy and didn't work very well. Looks like Go has been ported in, as well, but it doesn't support wifi or bluetooth on the esp32. Or even i2c or pwm.