I planted some seeds knowing that (1) they were a commercial crop, so quick growing; (2) I didn't have any possibility of growing them into maturity as I lack the space; and (3) I would never fruit them as I didn't have the heat!
So this was just an experiment.
My conclusion. They do grow extremely quickly and, if I hadn't deliberately constrained them, would have behaved like weeds.
The first constraint was that I planted 10 seeds in a 7cm pot. They need vastly more root space than that. The second was that I covered the pot with a plastic bag (of a type intended for keeping food in a fridge).
The growth was so strong that the plants burst out of the bag. The stems seem to be thicker, more whiskery (though this is maybe just because they are new and deciding what to do) and with more aerial roots than the stems inside. I don't know whether this was because the act of constraint encouraged the resultant growth, or whether the light outside the bag provided more of a stimulus.
The plants produced a lot of aerial roots. I know that such roots are usual for dragon fruit, more to climb than to root - but perhaps in this case the roots were encouraged additionally by poor nutrition or light?
The first and second pictures below show the seedlings inside the plastic bag. You can see some of the heroic escapes.
The third picture shows a break-out at the top. The new stem is much more whiskery than the stem in the bag.
The fourth picture below shows two seedlings that have burst out of the bag. One (or maybe both) has strangled the other with an aerial root.
The last picture shows the stems unconstrained.