If Progressbar in 'indeterminate' mode run too fast or too slow then you can use after(milliseconds, function_name) to run periodically own function which will use step() to change value in ProgressBar.

If you use different milliseconds or different value in step() then it will move with different speed.

import tkinter as tk
from tkinter import ttk

def change():
    p.step(10)
    root.after(100, change) # run again after 100ms,

root = tk.Tk()

p = ttk.Progressbar(root, mode='indeterminate')
p.pack()

change() # run first time 

root.mainloop()

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