to slow frame capturing

I use simple AverMedia card with BT878 chip.

I need to capture frame by frame and process each. I try to do this with the 
following code, but I get only 12 frames per second. I need at least 20 fps, 
but I don't what is wrong with my code, or what needs to be change, to get 
frames more frequently. 

Is it a matter of coding or the chipset?

I would be very thankful if somebody could help me.
Jacek

///////////////////
//capture code

dev.Open("/dev/video0");

//get memory buffer size
dev.GetMemoryBuffer(&stMemBuf);

//map memory for frame buffer
szFrame = (char*) mmap(0,stMemBuf.size,PROT_READ|PROT_WRITE,MAP_SHARED,dev,0);

//set channel to COMPOSITE and mode to PAL
dev.SetSource(1,VIDEO_MODE_PAL);

//set capture image size and pixel format
stMemMap.width = IMAGE_WIDTH;
stMemMap.height = IMAGE_HEIGHT;
stMemMap.format = VIDEO_PALETTE_GREY;

//capture frame 0
stMemMap.frame = nFrame;
ioctl(dev,VIDIOCMCAPTURE,&stMemMap);

while (m_bProcessing)
{
  stMemMap.frame = nFrame;
  ioctl(dev,VIDIOCSYNC,&stMemMap.frame);

  stMemMap.frame = 1 - nFrame;
  ioctl(dev,VIDIOCMCAPTURE,&stMemMap);
		
  ProcessFrame(..); //actually do nothing, simply count up frames

  nFrame = 1 - nFrame;
}