You don't actually need to "split" anything, you just read from different offsets per thread. Mmap might be the most efficient way to do this (or at least the easiest)

Whether or not that's going to run into hardware bottlenecks is a separate issue from designing a parallel algorithm. Idk what OP is trying to accomplish, but if their hardware is known (eg this is an internal tool meant to run in a data center), they'll need to read up on their hardware and virtualization architecture to squeeze the most IO performance.

But if parsing is actually the bottleneck, there's a lot you can do to optimize it in software. Simdjson would be a good place to start.

I think, you could open the same file multiple times and then just skip ahead by some number of bytes before you start reading.

But yeah, no idea if this would actually be efficient. The bottleneck is likely still the hard drive and trying to fit multiple sections of the file into RAM might end up being worse than reading linearly...