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The only other game-related causes thing I can think is bad drive, low space remaning on drive, or anti-virus software...
At runtimem the engine mounts the big .pak file, reads compressed files from them as needed (doesn't load the whole file, just files as needed from inside it), uncompresses them, and puts them in memory.

If there's something completely unrelated to the game causing it, no idea.

I'd recommend closing all unnecessary applications and taking a look at task manager to see if anything maxes out.

If so, you might just have too low specs.

But this probably isn't the case. Good luck!