I would be more likely to work the OTHER way: have your adventure, with a minimum number of encounters - 3 or 4, whatever you think is the fewest you can use to make an adventure work. Then have 2-3 extra encounters or puzzles or traps or whatnot (an interesting NPC?) to insert, if they have made it farther, faster than you expected. In other words, if you have 3 hours to play, and you have 3 scenes or encounters planned, and after the first hour, they've finished scene 2, and you realize you're probably going to finish before your alloted timespan is up, then drop in the puzzle you had prepped but not required for the door into the final lair... or toss in an extra combat with a wandering orc guard who just happens to be passing by at the last moment. That way, if they are dragging and lollygagging around, you can omit that encounter and speed them on their way. NEVER be afraid, in a one-shot, to say "and then, after an hour of (whatever), you reach...(the final destination)". Just the same way you STARTED in the middle of the action, with a one-shot, sometimes it is necessary to condense things that, in a campaign, would be valuable side- or back-story.