The way videogame quests work is that essentially everything is like eight different things. Commander Shepard jokes about this in Mass Effect 2, but basically in order to accomplish the one thing you are there for - say, recruiting a scientist to your ship - you have to do eight other things that are tangentially related to that goal in order to accomplish it. Sometimes the eight things are directly related. Sometimes they are just clearing up something so an NPC can focus. It's not completely unnatural.
If the PCs are looking to make a vaccine to some plague, then it makes sense they'd need to collect the bits needed for the scientist to put things together, right? If an NPC needs to be recruited, it makes sense that they may have some business that needs to be finished before they can go.
More Stuff = More Time
The more sub-quests involved in a given task,the more time it takes. Videogame makers love this because it's all content. Your story doesn't need to have as many major beats to it if the quests that make up those major beats each have a number of their own minor things. So what could be 2 hours of hopping around the galaxy getting a team together can instead be 5 hours or 10 hours of doing the small tasks related to getting that team together. Which saves you from needing 8 hours more content of what happens next because it's already taken care of.
Not Off The Hook Narratively
That doesn't mean these subquests just give you a pass. I mean, it can. However, if you don't treat the subquests with respect regarding setup and narration then it's going to become obvious you're padding things out for length, and that can be the difference between entertained players and ones just going through the numbers. If you can't make the parts compelling in their own right, they shouldn't be there. It can save you from needing more content for the main part, but that doesn't mean it's not content that shouldn't have it's own worth.
With those two things together, controlling duration should be obvious. If you need to run the clock out a bit - say you are missing a key player for the big fight but people are trucking through - then you have less ready for the PCs in game. That vaccine they need? Yeah, the doc needs blood samples and the PCs need to go get them. Each one has to be hunted down separately.
On the other hand, you need to wrap things up or you're going to be cutting session in the middle of a big event? More is already in place. The vaccine is ready to go. Once the PCs arrive they can be innoculated and go on with their bad selves.
You can also cut in the middle of things. Did that first sub quest take longer than expected at the table? Maybe a different group or something else brought in the others while they were out. Or the doc can make do with just that sample as it is particularly viable.
Have fun with it. Use it to make the game and world feel more real and responsive. Use it to make sure your PCs are where they should be XP wise when they get to that encounter. Or don't. Your call.