I can assure you that GRBL works by generating pulses for each axis simultaneously. I have read the code! I can't be certain for other motion controllers, but as an architecture it makes sense. After all, the output connections are in parallel and there is no electrical reason why one pulse must follow another rather than happen at the same time. What I don't understand is why things like the CSMIO can run so fast, much faster than necessary, if it is only used for steppers. Perhaps because of the custom hardware in the device?

Happy to continue the discussion when we have both had a night's sleep!