As much as I remember classic Who fondly, the pacing is sooooo slow now that I've seen the modern show.
Well, remember, these were meant to be watched in 25-minute installments. It seems slow-paced when you watch the whole serial in one go, because of the repetition and recaps and multiple inconclusive climaxes. But I imagine if you only saw one half-hour installment per week, the pace would feel better.
Personally I didn't find "Pyramids of Mars" to be slow-paced at all. It went along rather briskly compared to some, and the strong writing and character moments helped maintain interest.