Sure. The problem is that intermediate-skill developers read comments like yours and just glean "lol, syscalls are slow" from them. They then go on to use mmap

Sure. The problem is that intermediate-skill developers read comments like yours and just glean "lol, syscalls are slow" from them. They then go on to use mmap and kill MM performance. It's important to keep your audience in mind. Most developers do not write high-performance IO layers that talk directly to hardware. Most developers will blindly follow any sufficiently authoritative voice that tells them X is better than Y, which leads them to misuse powerful tools. In the vast majority of cases, you want plain read and write. In the vast majority of cases, the overhead of jumping into the kernel is not your bottleneck. Simplicity counts for a lot.