I think with Three it was a Budgetary Decision to do longer stories to save money (like two-parters in Nu Who, I suppose) and some of them end up very very padded with nothingness. Planet of the Spiders is a story I like but it has one episode that's literally just an extended car chase in a variety of vehicles. When I watched Ambassadors of DEATH recently it was slow in an interesting way for the most part but there's so much escape-and-be-recaptured and stuff happening just to slow down the plot. I have no patience, sigh.