There can be a huge difference between scheduled turnaround and actually achieve time. It should be up to the captain to take a good part in the management of the turnaround if a short one is desired.
Assuming a B737 / A320 type, it would be foolish to schedule 30 minutes at an airfield with shortage of handling staff or equipment with possible wait for refuelling. On the other hand, if there is a turnaround where minimum effort is required, perhaps re-fuelling not needed, small numbers of disembarking and joining pax, no freight etc a shorter turnaround could be scheduled.
There are ways to reduce turnaround time, like fuel tankering to minimise the time needed for any necessary re-fueling, keeping through pax on board and re-arranging the way cleaning is conducted.
We have all experienced fast turnarounds at times, with a bit more consideration and effort when running out of crew duty time etc. At these times we realise what can be achieved. I have seen a safe 6 minute DC9-30 turnaround at a country airport with a small refuel in 6 minutes. Also a pre-planned 10 minute B727-200 turnaround without refuelling with centre engine running.
Safety is always the major consideration at such times and I considered the 6 minute turnaround met all the then existing safety requirements, but I was less satisfied with the 10 minute one due I do not like the idea of continued engine operation.