I may be misremembering a drunken conversation with a developer but IIRC the root cause was choice of cross-platform APIs available in early 2010s & the JSON file was tiny when introduced.
The failure is that loading times had been a complaint for years, and nobody involved lifted a finger. It would be impossible to use the platform without feeling the pain.
The software was released on 7 platforms, not counting multiple Windows versions. I don't know the risks or what platforms changes impact today or the test effort involved. I expect "it's still functioning as expected" was the default.