Although this is possibly true, at any time the dev team could've gone: "loading is slow man, can we just profile it and see if there's anything obvious?". To someone with access to the source code and a debugger, that's probably less than 30 minutes of time to go from zero to hero.

I've done this kind of stuff many times, and something like a json array taking minutes to parse would likely be very very obvious when looking at a trace.