In the 1970's all our BAC 1-11 and F27 turn-rounds were scheduled as 25 minutes. In my area we handled some 25-30 a day at 3 airports.
Typically, about 30% of passengers would remain on board in transit. Baggage volumes were quite large.
The ground equipment was of the technology of the time; much cruder than modern stuff.
Not all turn-rounds would include fuelling but if they did it was not a problem, partly, I suppose, because the quantities were small due to the short sectors.
I have no recollection at all of any problems stemming from turn-rounds being too short, or of crews saying that their job was difficult because of it. I always believed that the 25-30 minutes turn-round was an economically necessary industry standard for short-haul operations. A typical rotation would have 6 20 - 40 minute sectors, and 5 25-minute turn-rounds. An additional 10 minutes, say, on turn-rounds would cost dearly in terms of aircraft utilisation. I'm sure the same applies today.
Incidentally, I think that VC10s and L-1011s were allowed 45 minutes for a transit stop, but perhaps that's my failing memory at work.