This is exactly the solution we’re implementing in the next Farmduino:
The coprocessor will live alongside the Arduino MCU and communicate with it over SPI. It is an STM32 processor with timers, which, as you mentioned, should make the sampling rate a non-issue. In the future, it is possible we might eliminate the Arduino MCU and shove the whole firmware into the STM32 for simplicity and cost reasons. Though we’ll need to determine if it is worth doing that and sacrificing being in the Arduino family.
The first (current) Farmduino was primarily designed to have a better board layout and connectors that are more optimized for FarmBot rather than 3D printers. This makes setup of the v1.3 electronics much easier, which was a pain point for v1.2 people who had RAMPS. The first Farmduino didn’t significantly improve performance over RAMPS, only the ease of use.
The next Farmduino aims to increase performance through the features mentioned above.
As with the development of any project, we must tackle issues in stages. Even with the next Farmduino, and the ones after that, there will be things to improve. Each iteration will get better (more performant, more reliable, lower cost, smaller size, easier to work with, etc), but we can only improve so much with each version because we have to continue shipping products and releasing new designs frequently.