The speed of capturing images can be improved by choosing a smaller format image. However, there is a tradeoff in that as the image gets smaller, its quality will decrease. If you are simply viewing the image on the screen, the difference in quality may not be of concern. However, if you are assembling the images for printing, and significantly increasing the scale of the images, the printed picture may appear blocky from being scaled up too much.
If you are experiencing problems because the image in the preview window doesn't seem to keep up with the VCR image, increase the preview rate. If it still doesn't keep up, you are probably pushing the limits of your hardware. Try a smaller video capture window size.