pygame pygame.display.update() function decreases fps

Question

We're making a small game in python but we have some problems with the pygame pygame.display.update() function. Its dramaticly decreasing our fps rate (40.000 to 27) while we're only redrawing a single sprite. (Rendering the background and the player doesnt really matter, it doesnt drop more)

Are we doing something wrong or is it just python being not that fast?

def render():
  screen.blit(background, (0,0))
  fred.draw()

  pygame.display.update(fred.pos)

FPS 28

def render():
  #screen.blit(background, (0,0))
  fred.draw()

  pygame.display.update(fred.pos)

FPS 31

as you can see removing the background from the render process does only increase the fps a bit. but still a drop from 40.000 to 28 when only redrawing 1 or 2 sprites?

Full code:

import pygame, sys, time
from pygame.locals import *

pygame.init()

size = width, height = 1024, 500
fps = 60
speed = 10

beginTime = time.clock()

screen = pygame.display.set_mode(size)
background = pygame.image.load('/pathtosprite/background.png').convert()

class Fred(object):
  strength = 0
  pos = 0
  sprite = 0

  def __init__(self, strength, position):
    self.sprite = pygame.image.load('/pathtosprite/fred.png').convert()
    self.strength = strength
    self.pos = pygame.Rect(10, 10, position[0], position[1])

  def draw(self):
    screen.blit(self.sprite, self.pos)

  def walk(self, direction):
    self.pos = self.pos.move(direction)

def tick(): #doet de berekeningen per stap
  lastTime = time.clock()
  keys = pygame.key.get_pressed()

  if keys[K_UP]:
    fred.walk([0,-speed])
  if keys[K_DOWN]:
    fred.walk([0,speed])
  if keys[K_RIGHT]:
    fred.walk([speed,0])
  if keys[K_LEFT]:
    fred.walk([-speed,0])

  render()


def render(): #updatet het scherm
  screen.blit(background, (0,0))
  fred.draw()

  pygame.display.update(fred.pos)

def run(): #zorgt dat fps gehandhaafd blijft
  startTime = time.clock()
  ticks = 0
  frames = 0

  while 1:
    for event in pygame.event.get():
      if event.type == QUIT:
        sys.exit()
    now = time.clock()
    if now-startTime > 1/fps:
      ticks += 1
      print(ticks/(now-beginTime))
      startTime = now
      tick()

fred = Fred(10000000, (10, 10))    
run()

Answer 1

Doing from pygame.locals import * means time refers to pygame's time module instead of the standard library time module.

Answer 2

You could use:

pygame.display.flip()

^is a bit faster than update. Also I recommend using delta time for moving if you add it to the game.