[Orokin Moon - Speed Test]

I'm pretty sure I did all my challenges in Plato (Crossfire Exterminate) after reading somewhere that in this mission you have a better chance to spawn more puzzle rooms (not really sure if that's accurate, but I often got 3 rooms in a single mission there). The speed challenge is by far the rarest in my experience, it's probably going to take a while before you see it.

Edit: to add further confirmation, I just found a speed test on Plato while doing the Nightwave challenge. Try there, and good luck!