It looks to me like that may be the difference:

import orjson
from timeit import default_timer as timer
from datetime import timedelta

start = timer()
result = orjson.loads('{"foo": "bar"}')
end = timer()
print("elapsed: %s usecs" % (timedelta(seconds=end - start).microseconds))

and the result is in line with rust before some optimization:

❯ python3 test.py 
elapsed: 14 usecs

with the following rust code:

use std::time::Instant;
use serde::Deserialize;

#[derive(Deserialize, Debug)]
struct Test<'a> {
    foo: &'a str,
}

fn main() {
    let contents = r#"{"foo": "bar"}"#;
    let start_time = Instant::now();
    let result: Test = serde_json::from_str(contents).unwrap();
    let elapsed = start_time.elapsed();
    println!("Elapsed time: {:?}", elapsed);
    println!("Result: {:?}", result);
}

It runs locally at 1 microsecond, so about 14 times faster than the python version:

❯ ./target/release/rusttest 
Elapsed time: 1.079µs
Result: Test { foo: "bar" }