I have had good results capturing 320x240 videos using the Component Video codec. It digitizes in 16-bit YUV format. I don't think there is any compression being applied, so the speed of your processor has very little to do with performance. IIRC, I was capturing ~30fps on a Quadra 840av when I had a fast hard drive installed (Quantum Viking II).

The MJPEG and JPEG codecs use compression, which adds CPU overhead. Those are best suited to capture cards that have on-board hardware codecs (ie. VideoVision Studio).

If you want to keep interlacing intact, try increasing the height to 480 (576 for PAL). The proportions will be off, but you can stretch it back out to full screen during playback to fix that.