There are two reasons: 1) The println! macro will lock and unlock the stdout every time it's called. 2) Rust's stdout is line-buffered by default.

In my machine the naive C implementation achieves 81.7 MiB/s, whereas a naive Rust implementation with println achieves 7.58 MiB/s.

When I acquired the stdout's lock once and use writeln!(&mut stdout, ...) instead of println, I got 8.15 MiB/s.

When I wrapped stdout with BufWriter, I got 299 MiB/s.

    use std::io::{BufWriter, Write};

    fn main() {
        let stdout = std::io::stdout();
        let stdout = stdout.lock();
        let mut stdout = BufWriter::with_capacity(32 * 1024, stdout);

        for i in 1..1_000_000_000 {
            let _ = match (i % 3 == 0, i % 5 == 0) {
                (true, true) => writeln!(&mut stdout, "FizzBuzz"),
                (true, false) => writeln!(&mut stdout, "Fizz"),
                (false, true) => writeln!(&mut stdout, "Buzz"),
                (false, false) => writeln!(&mut stdout, "{}", i),
            };
        }
    }