You guys should check your HDD/SSD usage in the task manager when it’s happening. You will likely see that it’s under very heavy usage and that your CPU might have some usage going on as well.
Basically, it’s analyzing and probably reorganizing the contents of the pak files to prepare for the actual download and to do it more efficiently. Think of the pak files(they are numerically labeled in the folder like data.35 and so on, but they are what most would simply call a pak file) like a mini harddrive spread out over the ~100 <=1gb pak files in the data folder, and then running a “defrag” on it. There can be a lot of empty space in them, so moving things around to minimize the space between assets, means you can have larger contiguous empty chunks for assets that might need more space; in order to avoid having to fragment them or having to add more *.pak files to the list. While doing that, it has to keep updating the registry of them all in the *.idx files(assuming that’s what they are, based on their small sizes).
Long story short, you have a slow drive, a weak CPU or some kind of security settings are slowing down the IO process of it all.