Target shooter game lags terribly when firing bullets

Question

I am programming a small game which is a target shooter for a school project. If you run the game, you will notice that the targets already move with a sort of stutter but then when you start firing bullets, it becomes unplayable. Is this just the framerate pygame offers or is it something that can be fixed in the code?

pygame.init()
window = pygame.display.set_mode((800, 575))
pygame.display.set_caption("TARGET PRACTICE")

#Colour variables (Module 1)

BLACK = (0, 0, 0)
WHITE = (255, 255, 255)
RED = (200, 0, 0)
GREEN = (0, 200, 0)
BLUE = (0, 0, 200)


exec = True

#Target class created (Module 5)

class Target:
  def __init__(self, x, y, h, w, v):
    self.x = x
    self.y = y
    self.h = h
    self.w = w
    self.v = v


#Instantiation of targets (Module 5)

target_1 = Target(0, 80, 60, 40, 0.5)
target_2 = Target(0, 100, 60, 40, 0.5)
target_3 = Target(0, 50, 60, 40, 0.5)
target_4 = Target(0, 75, 60, 40, 0.5)
target_5 = Target(0, 45, 60, 40, 0.5)
target_6 = Target(0, 85, 60, 40, 0.5)


#Declaring variables to be used in the while loop (Module 5)
clock = 0

target_2_threshold = 500
target_3_threshold = 1000
target_4_threshold = 1500
target_5_threshold = 2000
target_6_threshold = 2500


#Setting player sprite dimension variables (Module 6)

player_sprite_x = 357.5
player_sprite_y = 450
player_sprite_h = 125
player_sprite_w = 85

all_bullets = []

while exec:
  pygame.time.delay(1)
  for event in pygame.event.get():
    if event.type == pygame.QUIT:
      exec = False
    if event.type == pygame.MOUSEBUTTONDOWN:
        if event.button == 1:
            print("[shoot!] mouse position:", event.pos)
            dx = event.pos[0] - (player_sprite_x+ player_sprite_w//2)
            dy = event.pos[1] - player_sprite_y
            direction = pygame.math.Vector2(dx, dy).normalize()
            bullet = {'x': player_sprite_x+42, 'y': player_sprite_y, 'direction': direction}
            all_bullets.append(bullet)

  #Defines movement of targets and sets delay between drawings (Module 5)   

  clock += 1
  target_1.x += target_1.v
  if clock > target_2_threshold:
        target_2.x += target_2.v
  if clock > target_3_threshold:
        target_3.x += target_3.v
  if clock > target_4_threshold:
        target_4.x += target_4.v
  if clock > target_5_threshold:
        target_5.x += target_5.v
  if clock > target_6_threshold:
        target_6.x += target_6.v

  all_bullets_keep = []

  for item in all_bullets:
    item['x'] += item['direction'][0] # item['direction'][0] * 2
    item['y'] += item['direction'][1] # item['direction'][1] * 2
    # keep bullet if it is still on screen
    if 0 < item['x'] < 800 and 0 < item['y'] < 575:
          all_bullets_keep.append(item)

  all_bullets = all_bullets_keep
  #print(len(all_bullets), end='\r')

  #Fill the background (Module 5)

  window.fill(RED)

  #Redraw each target in every frame (Module 5)

  pygame.draw.rect(window, BLUE, (target_1.x, target_1.y, target_1.h, target_1.w))
  if clock > target_2_threshold:
      pygame.draw.rect(window, BLUE, (target_2.x, target_2.y, target_2.h, target_2.w)) 
  if clock > target_3_threshold:
      pygame.draw.rect(window, BLUE, (target_3.x, target_3.y, target_3.h, target_3.w))
  if clock > target_4_threshold:
      pygame.draw.rect(window, BLUE, (target_4.x, target_4.y, target_4.h, target_4.w))
  if clock > target_5_threshold:
      pygame.draw.rect(window, BLUE, (target_5.x, target_5.y, target_5.h, target_5.w))
  if clock > target_6_threshold:
      pygame.draw.rect(window, BLUE, (target_6.x, target_6.y, target_6.h, target_6.w))

  for item in all_bullets:
    pygame.draw.rect(window, BLUE, (item['x']-5, item['y']-5, 10, 10))

  #Draw the player sprite (Module 6)

  pygame.draw.rect(window, BLUE, (player_sprite_x, player_sprite_y, player_sprite_w, player_sprite_h))

  pygame.display.update()


pygame.quit()