Recursive factorial calculations in Rust

Question

I am trying to calculate very large factorials. I am looking to speed it up as 10000! takes approximately a minute and a half.

extern crate num;
use num::bigint::BigInt;
use num::bigint::ToBigInt;

fn tree_fact(n: u64) -> BigInt {
    if n < 2 {
        return 1.to_bigint().unwrap();
    }
    return factorial(1, n)
}


fn factorial(low: u64, high: u64) -> BigInt {
    if low +1 < high {
        let mid = (low + high) / 2;
        return factorial(low, mid) * factorial(mid + 1, high)
    }
    if low == high {
        return low.to_bigint().unwrap();
    }
    return low.to_bigint().unwrap() * high.to_bigint().unwrap();

}


fn main() {
    let fac_value = tree_fact(10000);
    println!("{}", fac_value);
}

I am also concerned with following best practices as I am new to programming in Rust so feel free to point out where I am bending any rules.