OTA channel surfing is slow because before you can see a picture, first you have to tune the digital signal, then you have to wait for an I frame, plus whatever decoder delay and output image processing. In order to make it better, you really need to three tuners, so you can process the streams of the up and down channels. Random access would still be slow, but oh well.

I imagine cable is pretty similar, but I suspect most cable boxes are more capable than tuners in tvs, so they may be running multiple tuners, and I think there's more of a chance of subchannels being useful, too.