Whenever a TV receives a non-native resolution, it has to scale/de-interlace the image before displaying it. For example, if you have a 1080p TV, and watch CBS, it has to deinterlace the 1080i signal. If you change the channel to Fox, it has to scale the 720p image to 1080p before displaying it.
I own a Sharp LCD and a Sony SXRD, it usually takes a few seconds to adjust to a different resolution. If you have a cable/satellite box, you can set it to output a specific resolution, which will be much quicker than letting the TV scale the resolution. The PQ usually not as good when the box does it though.