Slowness in Picamera2 while saving image

[waikeat2397]

Just to have a discussion with Raspberry Pi enthusiast here, might not really related to Picamera2.

I am trying to capture full quality image as fast as possibile, while saving the image in threads.

import time
import random 
from picamera2 import Picamera2
from libcamera import controls
import cv2
import threading
import io
from PIL import Image
import base64

def save_image(image):
#    time.sleep(1)
    cv2.imwrite("test.png", image, [cv2.IMWRITE_PNG_COMPRESSION, 0])
#    cv2.imwrite("test.png", image, [cv2.IMWRITE_JPEG_QUALITY, 95])

picam2 = Picamera2()
camera_config = picam2.create_still_configuration(main={"size": (4608, 2592), 'format': 'RGB888'}, raw={"size": (4608, 2592)})

picam2.configure(camera_config)

picam2.start()
time.sleep(1)

while True:
    t = time.time()
    image = picam2.capture_array()
    print("Capture time: ", ((time.time() - t) * 1000))
    x = threading.Thread(target=save_image, args=(image,))
    x.start()

Running script above, when I am not saving image, capture image time is at average 270ms very constant. But if I enable save image, capture image time become fluctuate between 350-600ms.

I tried running in different raspberry pi version, different sd card, emmc, or ssd all perform the same.

Anyone has experience with this? Any ways to optimise the performance?