Pre-allocation of memory for a string

In this example I am trying to show how pre-allocation of memory for a string can improve speed. However in this example I can only see 3% time difference which I think is meaningless.

examples/pre-allocation/src/main.rs

macro_rules! prt {
    ($var:expr) => {
        println!(
            "{:10} {:10} {:p} {:15?}",
            $var.len(),
            $var.capacity(),
            &$var,
            $var.as_ptr(),
        );
    };
}

fn main() {
    let len = 100_000_000;
    {
        let start = std::time::Instant::now();
        let mut text = String::new();
        
        for i in 0..len {
            text.push_str("x");
            let _t = String::from("x"); 
            if i == 0 {
                prt!(text);
            }
        }
        prt!(text);
        println!("{:?}", start.elapsed());
        println!("len: {}", text.len());    
    }

    println!();

    {
        let start = std::time::Instant::now();
        let mut text = String::with_capacity(len);
        for i in 0..len {            
            text.push_str("x");
            let _t = String::from("x");
            if i == 0 {
                prt!(text);
            }
        }
        prt!(text);
        println!("{:?}", start.elapsed());
        println!("len: {}", text.len());
    }

}

examples/pre-allocation/out.txt

         1          8 0x7ffe80bc9d20  0x64c4d419bb80
 100000000  134217728 0x7ffe80bc9d20  0x704611600010
11.727765301s
len: 100000000

         1  100000000 0x7ffe80bca4e0  0x70461b800010
 100000000  100000000 0x7ffe80bca4e0  0x70461b800010
11.367938399s
len: 100000000