You must have a loop in your code where you are doing all the video processing.

Let's say you have something similar to this pseudocode:

//code initialization
cv::VideoCapture cap("some-video-uri");

//video capture/processing loop
while (1)
{
        //here we take the timestamp
        auto start = std::chrono::system_clock::now();
        //capture the frame
        cap >> frame;
        //do whatever frame processing you are doing...
        do_frame_processing();
        //measure timestamp again
        auto end = std::chrono::system_clock::now();
        //end - start is the time taken to process 1 frame, output it:
        std::chrono::duration<double> diff = end-start;
        std::cout << "Time to process last frame (seconds): " << diff.count() 
                  << " FPS: " << 1.0 / diff.count() << "\n";
}

thats it ... take into account that calculating FPS in a frame-per-frame basis will likely produce a highly variant result. You should average this result for several frames in order to get a less variant FPS.

The recent versions of opencv-python will give an error called AttributeError because cv2 doesn't have any attribute named cv.

Instead use the following

import cv2

vidcap = cv2.VideoCapture('some_video.avi')
fps = vidcap.get(cv2.CAP_PROP_FPS)

print(f"{fps} frames per second")

This will give the frames per second value