speed detection using optical flow

I was referring to this mathwork's site...

Live motion detection using optical flow

for finding the speed of objects from a video file I have. But the code didn't say anything about how to find a speed. Also when I run the code I could see no optical flow thing...can anyone help me...I must be doing something wrong. Here is the code (the main code is same as that shown in link, just a little addition of reading video file from the disk):

nFrames=0;

opticFlow = opticalFlowHS;

v = VideoReader('Activity1.avi');

while hasFrame(v)     % Process for the first 100 frames.

% Acquire single frame from imaging device.

frameRGB = readFrame(v);

% Compute the optical flow for that particular frame.

flow = estimateFlow(opticFlow,rgb2gray(frameRGB));

imshow(frameRGB)

hold on
drawnow
plot(flow,'DecimationFactor',[5 5],'ScaleFactor',25)

hold off

% Increment frame count

nFrames = nFrames + 1;

end

I tried 'drawnow' command as suggested here Only I could see the video but not the optical flow...

I couldn't attach the .avi file here otherwise I would have.

[Nachiket Patki]

Ok I could solve the problem... I use

frameWithFlow = getframe(gca);

imshow(frameWithFlow.cdata)

now I am getting the optical flow over each frame.

But still, there is one problem the video gets very slow how can I make it run with normal video speed? (or at least faster than the current one)