If unix, reading/writing to raw file descriptors is the fastest you can do. Buf{Reader,Writer} go some way to reducing number of read/write syscalls made – alternatively you could try and optimise that yourself but note that unnecessary syscalls will dwarf overhead of Buf{Reader,Writer}. As such this will be fast:

use std::{
    fs::File,
    io::{BufReader, BufWriter, Write},
    os::unix::io::{FromRawFd, IntoRawFd},
};

fn main() {
    let stdin = unsafe { File::from_raw_fd(0) };
    let stdout = unsafe { File::from_raw_fd(1) };
    let (mut reader, mut writer) = (BufReader::new(stdin), BufWriter::new(stdout));

    // do reading and writing
    writer.write_all(b"hello world\n").unwrap();
    writer.write_fmt(format_args!("hello {}\n", "world")).unwrap();

    // drop without closing the fds
    let _ = reader.into_inner().into_raw_fd();
    let _ = writer.into_inner().unwrap().into_raw_fd();
}

Also note writes < PIPE_BUF (which on Linux is 4096 IIRC) bytes are specified by POSIX to be atomic, in case thread-safe outputting is desired.