如何在不影响其他功能的情况下减慢循环中的一个功能？

我正在制作一个游戏，其中一艘船必须躲避即将到来的外星人。游戏的主类有一个功能，可以将一个新的外星人添加到组中。我想减慢这个功能，但是当我添加 sleep 命令时，整个循环变慢了。我需要外星人在给定的时间间隔出现。

import sys

import pygame

from time import sleep

from settings import Settings

from rocket import Rocket

from bullet import Bullet

from alien import Alien


import asyncio

class Cosmos:
    """Главный класс игры Cosmos"""

    def __init__(self):
        pygame.init()
        self.screen = pygame.display.set_mode((0, 0), pygame.FULLSCREEN)
        self.screen_rect = self.screen.get_rect()
        pygame.display.set_caption('Cosmos')
        self.fon_image = pygame.image.load('image/Fon.png')
        self.fon = self.fon_image.get_rect()
        self.settings = Settings()
        self.rocket = Rocket(self.screen, self.settings)
        self.bullets = pygame.sprite.Group()
        self.aliens = pygame.sprite.Group()

        self.alien = Alien(self.screen, self.settings)

    def key_event(self):
        """Обрабатывает нажатие клавиш"""
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()
            elif event.type == pygame.KEYDOWN:
                if event.key == pygame.K_ESCAPE:
                    sys.exit()
                elif event.key == pygame.K_d:
                    self.rocket.move_RIGHT = True
                elif event.key == pygame.K_a:
                    self.rocket.move_LEFT = True
                elif event.key == pygame.K_w:
                    self.rocket.move_UP = True
                elif event.key == pygame.K_s:
                    self.rocket.move_DOWN = True
                elif event.key == pygame.K_SPACE:
                    new_bullet = Bullet(self.screen, self.settings, self.rocket)
                    self.bullets.add(new_bullet)
            elif event.type == pygame.KEYUP:
                if event.key == pygame.K_d:
                    self.rocket.move_RIGHT = False
                elif event.key == pygame.K_a:
                    self.rocket.move_LEFT = False
                elif event.key == pygame.K_w:
                    self.rocket.move_UP = False
                elif event.key == pygame.K_s:
                    self.rocket.move_DOWN = False

    def spawn_alien(self):
        """Добавляет пришельца в группу"""
        alien = Alien(self.screen, self.settings)
        self.aliens.add(alien)


    def bullet_sprite(self):
        """Рисует и обновляет спрайт снаряда"""
        for bullet in self.bullets.sprites():
            if bullet.rect.x < 5000 and bullet.rect.x > 0 and bullet.rect.y < 1500 and bullet.rect.y > 0:
                bullet.draw_bullet()
                bullet.update()
            else:
                self.bullets.remove(bullet)

    def hui(self):


    def run_game(self):
        """Обновляет события игры"""
        while True:
            self.screen.blit(self.fon_image, self.fon)
            self.key_event()
            self.rocket.update_rocket()
            self.bullet_sprite()
            self.rocket.draw_rocket()            
            self.spawn_alien()
            self.aliens.draw(self.screen)
            print(self.aliens)
            pygame.display.flip()


game = Cosmos()
game.run_game()

import pygame

from pygame.sprite import Sprite

from random import randint


class Alien(Sprite):
    """Класс, представляющий одного пришельца"""

    def __init__(self, screen, settings):
        super().__init__()
        self.screen = screen
        self.settings = settings
        self.image = pygame.image.load('image/pixil-frame-1.png')
        self.rect = self.image.get_rect()
        self.rect.x = randint(0, 1800)
        self.rect.y = randint(0, 1000)