how to improve the capture speed from V4L

[James REGIS]

hi,I am make program to capture from v4l device, when I use 640x480, it can't do 12.5 frame per second,this speed is common, with 320x240, it also do 12.5 fps,it's too slow, why? I compare with the fbtv in console mode, 640x480 it can do 12.5 fps, the same as mine, but with resolution 320x240,it can do 24~25 fps, the source code of fbtv is fairly complex and now I have no much time to read it. can anyone give me a sugguest to improve the capture speed! I attache my code for ref .thanks!