In-kernal block caching of files is pretty crude - since the kernel has no knowledge of your data structures or which bits of your files you're going to be reading next, you end up having hundreds of page faults requiring single-sector disk reads for most workloads.

That’s often the case if your file format evolves without taking a mmap use case into account. But for a format that’s designed with mmapping in mind, it is often ridiculously faster and more effective.

But most languages and environments don’t let you do that easily.