If you have infinite time, you can create a decoration of any kind and out of almost anything. I have seen the lakes with real water in the middle of the ball room (the lake was one inch dip) with a real bridge. One of my colleagues recreates famous places from around the globe: Trafalgar Square or Eiffel Tower. Everything is produced in their warehouse, loaded into trucks, unloaded on site, taken into the room and assembled. And then, in reversed order, after the party... Amazing work! But, can take a lot of time (and budget too!). If we have it.
But that is usually not the case. Usually the team of decorators is given a few hours before the party. If it is not enough for the production process, the client must pay for the rent of the space while decorators produce and install the props. Clients do not like it, surprisingly... But even worse, a lot of venues might be available on the day of the event, but sold to another group for the day before. So time necessary for installation must be carefully estimated during the planning process.
Yes, you can create something off site and bring it in and assemble it quickly. I would guess, the delivery and assembly process will eat up all possible advantages...
Balloon sculptures can be created much quicker than almost any other decorations. And, in a lot of cases, it is time - labor trade of. We have decorated a party that took place only one hour after previous one. So, we have brought a larger team and dressed up the room on time!
Is it an absolute rule? - No. It depends on the design. Once we have been asked to create a room out of strings of the balloons, suspended from the ceiling. Without going into technical details, such a creation would take, among other things, inflating tens of thousand of balloons. Thant takes time, no matter how many people will do it in parallel, there are only so much space to place the equipment, teams and inflated balloons. But, as ALWAYS there are several alternative designs to achieve the same effect with less resources. All we need is careful on-time planning. Which is not always the case: once in a while people call with a project like that two days before the event.
Start planning early! Call ahead of time and get a consultation! It is free!