It can be. Its usually better to have a bunch of prepared, prepackaged stuff to use for Plot, and improv the in between bits.
My setup for preparing a Shadowrun was 1) recap of the previous week, written as a news story for the players to read while I got everything organized 2) written out speech for the employer of the week, so the players knew what the mission was 3) a list of the locations they would go to and who was there to meet (I also sometimes created character sheets for important enough characters, and knew about how much opposition of what types they could/would face at different times) 4) any details/notes 5) page references for rules I knew I needed and 6) rewards for completion. This is usually 3/4 of a page of paper, total.
Number 3) is literally just a list, maybe with some description of the place. I'm not planning out how the players get there or what they do while they're there. My job ends at setting up a puzzle and some goals. The players are the ones who interact with it (and the entertainment I get is mostly from them coming up with stuff I hadn't considered). As long as I give them less-than-complete information or something isn't what they were expecting (I add a twist to what I told them), I only need to put together a goal and some places. It's less work, gives the players more options, and it's real hard to go off the rails when I didn't bother creating any.
To help with making stuff up on the fly, I make sure I have some knowledge about what's going on in the background. If you're attacking the half-finished death star that nobody is supposed to know about, what is security like? Probably an incomplete or not-yet-functional electronic security system, because it's not built. Security is probably made of soldiers, because it's a military base, and they're probably stationed there long-term instead of rotated out because it's supposed to be a secret. You could justify having a large amount of security personnel because it's a military base, but it might make sense to have few because housing is also unfinished and secrecy is easier with fewer people. I'd want to make a decision on that beforehand. The builders being everywhere means you're more likely to be seen trying to do something, but people will be less curious about weird noises because construction involves weird noises (visual perception will be in play a lot more than auditory, and the players would have more success blending in than trying to sneak around). Also, the builders are just there for the money, so as long as it doesn't affect them directly, they aren't overly concerned what the players are doing. So if the players decide to blow their way in, the explosion will get less attention than it would elsewhere, but I would check to see if anyone saw them walking in through the hole, and if they care that they saw the players (anything that will cause them more work or put them in danger/get them yelled at). If they decide to pretend they're pizza delivery people delivering to the secret base, they're going to do poorly. If they want to blow the whole thing up, I would consider that it's a giant military base with a giant laser on it, which
would have a large reactor to power it, so they would be able to find a thing to blow up. If they want to steal everything, they could get some welding equipment, but not much else of value, because it's a partially-finished base. Knowing that Vader is coming to visit means everyone is working harder, and paying less attention to what you are doing. But, if you look like you aren't working, you're going to get people upset. You can make a lot of decisions based on what a place is, and what people's goals are.