Hehe. Optimization ... it's a good way to learn. Earlier in my career I did a lot of PHP. Usually close to bare.

Other than the obvious point that writing an enormous JSON file is a dubious goal in the first place (really), while PHP can be very fast this is probably faster to implement in shell with sed/grep, or ... almost certainly better ... by loading to sqlite then dumping out from there. Your optimization path then likely becomes index specification and processing, and after the initial load potentially query or instance parallelization.

The page confirms sqlite is available.

If the judges whinge and shell_exec() is unavailable as a path, as a more acceptable path that's whinge-tolerant, use PHP's sqlite feature then dump to JSON.

If I wanted to achieve this for some reason in reality, I'd have the file on a memory-backed blockstore before processing, which would yield further gains.

Frankly, this is not much of a programming problem, it's more a system problem, but it's not being specced as such. This shows, in my view, immaturity of conception of the real problem domain (likely IO bound). Right tool for the job.