Python - How to speed up smoothness in pygame?

I'm wondering how to speed up the smoothness of my code written in Python using pygam. I'm guessing I have to make this more efficient somehow? When this is run, some balls move around randomly in a set area, however, the new position of each ball is not smooth at all, there is a jump between each movement as the cycle is very slow. How do I fix this? Or is there any suggestions on how to improve it? This is my code so far:

import pygame
from pygame import *
import random
pygame.init()
size = width, height = 800, 600
screen = display.set_mode(size)
pygame.display.set_caption("Year 12: Ideal Gas Simulation")


BLACK = (0, 0, 0)
RED = (255, 0, 0)
BLUE = (0, 0, 255)
WHITE=(255,255,255)
GREEN = (0, 255, 0)
BALLX = 0
BALLY = 1
BALLSPEEDX = 2
BALLSPEEDY = 3
List=[]
radius=5

running=True
myClock=time.Clock()


myClock.tick(60)
def initBall():


        for n in range(40):
            ballx = random.randint(0, 800) # randomly setting the x position
            bally = random.randint(0, 600) # randomly setting the y position
            dirx = random.randint(-5,5)    # randomly setting the x speed
            diry = random.randint(-5,5)    # randomly setting the y speed

            data=[ballx, bally, dirx, diry]
            List.append(data)
            # returning a list with all the data the ball needs
        return List # returning the list


def drawScreen(List):
    draw.rect(screen, WHITE, (0, 0, 800, 600))
    for x in range(40):
        BALLX=List[x][0]
        BALLY=List[x][1]
        draw.circle(screen, GREEN, (BALLX,BALLY),radius)
        display.flip()
        pygame.draw.rect(screen, BLACK, (100-radius,100-radius,600+(2*radius),400+(2*radius)), 1)

        f=pygame.font.SysFont(None,60)
        text=f.render("PV=nRT",True,(0,0,0))
        screen.blit(text,(300,height/20))


def moveBall(List):
    for x in range(40):
        BALLX=List[x][0]
        BALLY=List[x][1]
        SPEEDX=List[x][2]#####data[BALLX]== the first index of each list [x][0]
        SPEEDY=List[x][3]##data[BALLSPEEDX]= List[x][2]
        age=SPEEDX+BALLX
        List[x][0]=age
         # increases the position of the ball
        plus=SPEEDY+BALLY
        List[x][1]=plus
    # checks to see if the ball is hitting the walls in the x direction
        if BALLX > 700:
            List[x][0] = 700#NORMALLY 800
            third=List[x][2]
            answer=third*-1
            List[x][2]=answer
        elif BALLX < 100:#NORMALLY 0
            List[x][0] = 100
            third=List[x][2]
            answer=third*-1
            List[x][2]=answer

    # checks to see if the ball is hitting the walls in the y direction
        if BALLY < 100:
            List[x][1] = 100#NORMALLY 0
            third=List[x][3]
            answer=third*-1
            List[x][3]=answer
        elif BALLY > 500:
            List[x][1] = 500#NORMALLY 600
            third=List[x][3]
            answer=third*-1
            List[x][3]=answer
    return List#return updated list


List=initBall()
while running==True:
    for evnt in event.get():
        if evnt.type==QUIT:
            running=False
            quit()
        if evnt.type==MOUSEBUTTONDOWN:
            mx,my=evnt.pos
            button=evnt.button

    drawScreen(List)
    List=moveBall(List)