Low frame rate when running the Piston example

Question

Low frame rate when running the Piston example

I am building a simple 2D game in Rust using Piston. I used the examples from the Piston documentation and expanded it, and it works quite well. However, I get pretty poor performance:

Drawing just 2 squares gives me a frame rate of about 30-40 FPS
Figure 5,000 squares gives me a frame rate of around 5 FPS

This is on Core i7 @ 2.2GHz running Windows 10. Rust version 1.8, Piston version 0.19.0.

Is this expected, or have I made any mistakes in my code? Did I even measure FPS correctly?

extern crate piston_window;
extern crate piston;
extern crate rand;

use piston_window::*;
use rand::Rng;

fn main() {

    const SIZE: [u32; 2] = [600,600];

    const GREEN: [f32; 4] = [0.0, 1.0, 0.0, 1.0];


    const NUM: u32 = 1000; //change this to change number of polygons
    const SQUARESIZE: f64 = 10.0;


    // Create an Glutin window.
    let window: PistonWindow = WindowSettings::new("test",SIZE)
        .exit_on_esc(true)
        .build()
        .unwrap();

    let mut frames = 0;
    let mut passed = 0.0;

    let mut rng = rand::thread_rng();

    for e in window {

        if let Some(_) = e.render_args() {
            e.draw_2d(|c, g| {

                //clear the screen.
                clear(GREEN, g);

                for i in 0..NUM {

                    //setting up so that it looks pretty
                    let x = (i % SIZE[0]) as f64;
                    let y = (i % SIZE[1]) as f64;
                    let fill = (x / (SIZE[0] as f64)) as f32;
                    let color: [f32; 4] = [fill,1.0-fill,fill,fill];

                    let x = rng.gen_range::<f64>(0.0,SIZE[0] as f64);

                    //draw the square
                    let square = rectangle::square(0.0, 0.0, SQUARESIZE);
                    let transform = c.transform.trans(x-SQUARESIZE/2.0,y-SQUARESIZE/2.0);
                    rectangle(color, square, transform, g);

                }

                frames+=1;
            });
        }

        if let Some(u) = e.update_args() {

            passed += u.dt;

            if passed > 1.0 {

                let fps = (frames as f64) / passed;

                println!("FPS: {}",fps);

                frames = 0;
                passed = 0.0;
            }
        }
    }
}