Pathological performance with lots of arguments

[alexcrichton]

Reduced down from rust-lang/cargo#3547 (comment), the following program takes quite a long time to complete under --release

extern crate rustc_serialize;
extern crate docopt;

use docopt::Docopt;

// Write the Docopt usage string.
const USAGE: &'static str = "
Usage: cargo run [options] [--] [<args>...]

Options:
    -a, --archive  Copy everything.
";

#[derive(Debug, RustcDecodable)]
struct Args {
    arg_args: Vec<String>,
    flag_archive: bool,
}

fn main() {
    let mut args = vec![
        "cargo".to_string(),
        "run".to_string(),
        "--".to_string(),
    ];
    args.extend((0..20_000).map(|i| i.to_string()));

    let dopt = Docopt::new(USAGE).unwrap().argv(args);
    println!("starting");
    let parsed = dopt.parse().unwrap();
}

I'm curious, is this a "wontfix" in docopt or perhaps something that'd be relatively easily optimizable?