Experiences can vary, but I know that in recent builds of XBMC I've been playing with the biggest factor in time to tune has been XBMC sorting out the codec handlers for the stream.
For example I was testing with a rawts input (this is a debug mode) so there was no "tuning" involved, so TVH is essentially instantaneous (though obviously there will always be small delays while it sets things up internally).
XBMC took a noticeable (I didn't measure it) amount of time to tune the channel, whereas I also tried showtime (the frontend media system written by Andreas the original TVH author) and that was much quicker (at least to my eye).
Generally speaking on my main XBMC machine when I change channel TVH usually reports in the logs that its tuned and received the signal in <1s and XBMC appears to detect said incoming stream pretty quickly, however it still stalls a bit while getting its beans in a row (codec handlers).
But then of course, if you have a poor DVB card (or driver) then your experience may well differ. But generally speaking TVH is usually not the significant factor in either case (but doesn't mean it can't get things wrong occasionally ;) ).
Adam
