This is a very generic question because there are many different types of construction. I’ve never been involved with building houses but people I know that do that kind of work say that they could start a house and have it completely finished 4 or 5 days (if everyone shows up on time). What slows them down is that they have to stop work and get an inspector to approve the work done before the next task can be started. Depending on the jurisdiction you are in, that can add several months to the construction time.
There are similar issues with infrastructure construction. I’ve seen a roadway cutout and then left to weather 3 years before the engineer was satisfied that everything had settled in place. Also with big projects, manufactures will not order raw materials for your order unless you have a signed contract in hand. That can create a situation where you have to stop work because you need something that hasn’t been manufactured yet.
If a subcontractor shows up 3 months late, that adds time to the project. Even if every thing goes to plan, the various tasks involved have to been done in order and if the engineer says that the concrete has to cure for 60 days, you have to wait that time before the next thing can be started. I don’t want this answer to sound like I’m complaining about engineers or inspectors. If you don’t wait for things to be done correctly that can result in the death of the workers or the end users.