[Python] OpenCV Callback Lag

I'm running OpenCV 3 on Python 3.4 on a 2015 15" MacBook Pro. Below is a minimal example that illustrates my problem:

import cv2 as cv
import numpy as np

def mouse_callback(event, x, y, flags, param):
print("Callback!")

cv.namedWindow("Display")
cv.setMouseCallback("Display", mouse_callback)

cap = cv.VideoCapture(0)
while True:
ret, frame = cap.read()
cv.imshow("Display", frame)

if cv.waitKey(1) == ord("q"):
break


When I click on the screen, the text "Callback!" takes about 3 seconds to appear on the terminal screen. I'm not sure why I'm seeing this much lag--my laptop shouldn't be so bad that I can't even run this simple script.

Additionally, the problem persists when I reduce the webcam resolution, or even when I replace the webcam altogether with a still image. I rewrote a similar program in C++, and the C++ OpenCV libraries also suffer from this lag.

Any tips on how I can reduce or eliminate the lag?

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