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.
Again, you're not wrong, but none of those things are insurmountable. If the team is really so stressed it cannot spend 30 minutes, over the many year of the existence of that bug, that seems like a development environment close to hell.
Knowledge loss is precisely my point: there is very little a-priori knowledge needed to solve this, the guy who found the bug proves that.
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.