python script is slowly after use multiprocessing

[Xristos Xristoou]

i have create an image processing python function.

my system have 4 cores + 4 threads.

i want to use multiprocessing to speed up my function,but anytime to use multiprocessing packages my function is not faster and is 1 minute slowly. any idea why ?first time use multiprocessing packages.

main function :

if __name__ == '__main__':
    in_path="C:/Users/username/Desktop/in.tif"
    out_path="C:/Users/username/Desktop/out.tif"
    myfun(in_path, out_path)
time=3.4 minutes

with multiprocessing map :

if __name__ == '__main__':
    p = Pool(processes=4)
    in_path="C:/Users/username/Desktop/in.tif"
    out_path="C:/Users/username/Desktop/out.tif"
    result = p.map(myfun(in_path,out_path))
time=4.4 minutes

if __name__ == '__main__':
    pool = multiprocessing.Pool(4)
    in_path="C:/Users/username/Desktop/in.tif"
    out_path="C:/Users/username/Desktop/out.tif"
    pool.apply_async(myfun, args=(in_path,out_path,))
    pool.close()
    pool.join()
time=4.5 minutes