Recently I was looking at how to do streaming I/O in Swift, reading a file line by line (but not the whole file at once). In C on Linux, this is one function, fgets, which does the buffering itself and uses SIMD-accelerated memchr to look for the newline. You call it with a file handle and it's done.

In Swift the best advice I found was https://forums.swift.org/t/read-text-file-line-by-line/28852...: use the C functions. I experimented with using FileHandle in Swift and manually scanning the return Data value for newlines, and found it added a 6x slowdown. The loop for scanning the newline does not optimize even the array access, it was calling a lookup function for every byte and there was of course no SIMD acceleration. Converting the Data value to a native Swift array did at least eliminate most of the lookup overhead, but from the profile it seems like it still does bounds checking on every access, so it doesn't optimize to SIMD or anything like that.

So that's the issue - the Foundation library isn't a portable wrapper over the native system calls, it is a thick wad of gunk that adds significant overhead to even simple programs. There was an effort to make a better API, https://github.com/apple/swift-system/, but it seems to have stalled (or perhaps an improved cross-platform API was never its goal).