Why is this slow? It doesn't actually write the pixel...

code.py

from board import *
import time
import neopixel

class PixelPaint:

    def __init__(self, pixelobj):
        self.pixelobj = pixelobj
        self.n = len(self.pixelobj)

    def update(self):
        self.pixelobj.auto_write = False
        red = (30,0,0)
        starttime = time.monotonic()
        for j in range(0, self.n):
            pixelnumber = j
            self.pixelobj[j] = red
        stoptime = time.monotonic()

        total = stoptime - starttime
        print ("Timing total:", total)
        #self.pixelobj.fill((0,0,0))
        #self.pixelobj.show()

def run():
    with neopixel.NeoPixel(A0, 100, 3, 1, False) as np: # change to match pin and pixel count
        paint = PixelPaint(np)
        for i in range(0,100):
            paint.update()
            #np.show()

run()