Why does it take 15 minutes to open / load project?

I am creating a project for school which needs to be distributed and played by a large number of people, so I need load times to be quick. It was not a problem throughout the game's creation, but recently it is taking 15 minutes to open the project, and the same amount of time to load the scene when built. Still takes 10-15 minutes to load the built game on a high-power gaming computer. How did this happen, and any tips on how I can fix this? The file is not terribly large ( < 5GB) and I am happy to share it with anyone who can help. I admit I am not great with optimization, as I have never done this before now.