real-time acquiring and displaying image via camera

I am using a USB camera to acquire and display frames with some basic image processing (displaying a line on top).

when i write

preview(vid)

i can get a video preview of almost 10FPS dispalyed as video.

but when i code like:

loop start

start(vid);
trigger(vid); 
f=getdata(vid,1);
%insertshape to add line to image
imshow(f)

loop ends

the dispaly rate is very slow almost 1 frame per 1.6 sec.

How can I code to acquire and display images faster from my cam?