I built a JGRO design from MDF a few years ago. I won't talk about strength issues here, but using around 900mm M10 threaded rod as a leadscrew (with Delrin anti-backlash nuts) I find that I'm limited to around 900mm/min top speed on the X axis. This is down to leadscrew whip. The JGRO only has a single X leadscrew but that won't make any difference to leadscrew critical speed. If you want it to go faster, use a bigger leadscrew. Bigger diameter means higher rotational speed possible as well as coarser pitch for more speed at same revs. 3nm Nema 23 seems more than adequate to drive it.