Forge Mod Loading: Freezing Data?

[bobgrey1997]

I have been playing on 1.7.10 modded for many, many years. On 1.7.10, Forge has a rather odd-looking loading process, where it shows you the 7 phases of mod loading, then breaks each phase down into each mod and item that is loaded, and it shows you the progress of this on 3 rows of progress bars. I have grown quite accustomed to this.
The past few days, however, I have been playing on 1.15.2. It's quite the change, but I think I like it so far. There is one thing I found somewhat annoying, though. Upon launching a Forge pack, there is only one progress bar, which seems to progress rather smoothly right up until the end of it, where it looks to freeze. The game is not frozen, however, and the memory usage still fluctuates as normal. It is still responding and operating normally, it just looks to be frozen. It sits like this for some time. Being accustomed to the 3 rows of these progress bars (and having the little hammer and anvil in the bottom right doing it's animation to reassure you the program is not frozen), I find this change a bit unsettling. It's nothing too major though. However, upon staring at this screen for a bit, I did notice the bottom left corner, which at this stage reads "Freezing Data". Freezing data while the program appears to be frozen? Ironic, at least.
So, the main purpose of this post:
What does this "Freezing Data" actually mean? What is going on at this stage of loading?