Cargo.toml

[package]
name = "smol"
version = "0.1.0"
edition = "2021"

[dependencies.bevy]
version = "0.13.0"
default-features = false
features = [
  "multi-threaded",
  "bevy_winit",
  "x11",
]

[profile.release]
panic = 'abort'
strip = true
codegen-units = 1
lto = "thin"

main.rs

//! spawn/despawn many entities

use bevy::prelude::*;

#[derive(Component)]
struct F(f32);

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Update, de_spawn)
        .run();
}

fn de_spawn(
    keyboard_input: Res<ButtonInput<KeyCode>>,
    query: Query<Entity, With<F>>,
    mut commands: Commands,
) {
    if keyboard_input.just_pressed(KeyCode::KeyS) {
        for _ in 0..1_000_000 {
            commands.spawn(F(42.0));
        }
    }
    if keyboard_input.just_pressed(KeyCode::KeyD) {
        for e in query.iter() {
            commands.entity(e).despawn();
        }
    }
    if keyboard_input.just_pressed(KeyCode::KeyI) {
        info!("{}", query.iter().len());
    }
}