С временем игра начинает лагать

import sys
import pygame
from settings import Settings
from star import Star
from random import randint
 
class StarGame:
 
    def __init__(self):
        pygame.init()
        self.settings = Settings()
        self.screen = pygame.display.set_mode(
            (0,0), pygame.FULLSCREEN)
        self.settings.screen_width = self.screen.get_rect().width 
        self.settings.screen_height = self.screen.get_rect().height
        pygame.display.set_caption("Star Game")
 
        self.star_surf = pygame.image.load('images/alien.bmp')
 
        self.stars = pygame.sprite.Group()
 
        self._create_fleet()
 
 
 
    def run_game(self):
        while True:
            self._check_events()
            self._update_stars()
            self._update_screen()
 
            
    def _create_fleet(self):
        star = Star(self, self.star_surf)
        star_width, star_height = star.rect.size
        available_space_x = self.settings.screen_width - (2 * star_width) 
        number_stars_x = available_space_x // (2 * star_width)
 
        available_space_y = (self.settings.screen_height - 2 * star_height)
        number_rows = available_space_y // (2 * star_height)
        for row_number in range(number_rows):
            for star_number in range(number_stars_x):
                    self._create_star(star_number, row_number)
            
    def _create_row(self):
        star = Star(self, self.star_surf)
        star_width, star_height = star.rect.size
        available_space_x = self.settings.screen_width - (2 * star_width) 
        number_stars_x = available_space_x // (2 * star_width)
 
        available_space_y = (self.settings.screen_height - 2 * star_height)
        number_rows = available_space_y // (2 * star_height)
        for star_number in range(number_stars_x):
            self._create_star(star_number, 0)
 
    def _create_star(self, star_number, row_number):
        star = Star(self, self.star_surf)
        star_width, star_height = star.rect.size
        star.x = star_width + 2 * star_width * star_number
        star.rect.x = star.x
        star.y = star.rect.height + 2 * star.rect.height * row_number
        star.rect.y = star.y
        self.stars.add(star)
 
    def _update_stars(self):
        """Update the positions of all stars in the fleet."""
        self._check_fleet_edges()
        self.stars.update()
 
    def _check_events(self):
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()
            elif event.type == pygame.KEYDOWN:
                if event.key == pygame.K_q:
                    sys.exit()
 
    def _update_screen(self):
        self.screen.fill(self.settings.bg_color)
        self.stars.draw(self.screen)
        pygame.display.flip()
 
    def _check_fleet_edges(self):
        flag = False
        for star in self.stars.sprites():
            if star.check_edges():
                star.remove(self.stars) 
                flag = True
            if flag:
                self._create_row()
 
 
 
 
if __name__ == '__main__':
    sg = StarGame()
    sg.run_game()

import pygame
from pygame.sprite import Sprite 
 
class Star(Sprite):
 
    def __init__(self, sg_game, image):
        super().__init__()
        self.screen = sg_game.screen
        self.settings = sg_game.settings
 
        self.image = image
        self.rect = self.image.get_rect()
        self.rect.x = self.rect.width
        self.rect.y = self.rect.height
        self.x = float(self.rect.x)
        self.y = float(self.rect.y)
 
    def update(self):
        self.y += self.settings.star_speed
        self.rect.topleft = round(self.x), round(self.y)
 
    def check_edges(self):
        screen_rect = self.screen.get_rect()
        if self.rect.y >= screen_rect.bottom:
            return True

def __init__(self):
        """Initialise the game's settings."""
        #Screen settings
        self.screen_width = 1200
        self.screen_height = 800
        self.bg_color = (230, 230, 230)
 
        self.star_speed = 1