[Python] Why does this tkinter image animation not delay properly?

Here's my attempt, modified from another post, to create a minimal loop where I create a numpy array and display it. (For convenience I have a Start button.)

It works fine, with one exception -- it runs at full speed, regardless of what delay I put in as the first argument to self.after (it's 1000 in the example below. Changing this arg has no visible effect.)

What am I missing? (I am running python 3.8.3. Oh, that's apparently a very old version. Damn. I'll go update just in case that's related to the issue.)

from PIL import ImageTk
import numpy as np
import tkinter as tk

class App(tk.Tk):
def __init__(self):
super().__init__()
self.canvas = tk.Canvas(self, width = 1200, height=1200)
self.canvas.pack()
tk.Button(self, text="Start", command=self.start_animation).pack()

def start_animation(self):
self.canvas.delete('all')

# generate a random test frame
im = np.random.randint(0, 255, (1024,1024,3), dtype="uint8")
im1 = Image.fromarray(im, 'RGB')
im2 = ImageTk.PhotoImage(im1)

self.canvas.create_image(600, 600, image=im2)
self.update()
self.after(1000, self.start_animation())

if __name__ == '__main__':
App().mainloop()

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