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 problem was not in delivering JSON. There were better ways, but it was good enough.
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.
It would be dramatically easier to discover and exploit vulnerabilities/glitches in their multiplayer experience, which is their cash cow.