[Python] How to fast change image brightness with python + OpenCV?

I have a sequence of images. I need to average brightness of these images.

First example (very slow):

img = cv2.imread('test.jpg') #load rgb image
hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV) #convert it to hsv

for x in range(0, len(hsv)):
for y in range(0, len(hsv[0])):
hsv[x, y][2] += value

img = cv2.cvtColor(hsv, cv2.COLOR_HSV2BGR)
cv2.imwrite("image_processed.jpg", img)


Second example (quickly)

hsv += value


This example very fast but it changes all values HSV (I need to change only V (brightness))

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