Doctor Who has always had problems with pacing.
Back in ye olden times, when an episode would be made up of four 22-minute episodes, you'd get a structure like this
- 5-15 minutes building the setting and introducing characters, and a cliffhanger that reveals this episode with of the Daleks in the title has Daleks in it.
- The actual plot, interspersed with capture and escape segments.
- 15 minutes of plot resolution, and then the Doctor says goodbye.
And that's the good episodes. The bad ones, you could look forward to 6 episode slogs through the gravel quarries and welsh wastelands of the universe, with the Doctor and his companion getting caught and released every week for three weeks in the middle, plus a hovercraft chase.
In ye newen times, the problems aren't better or worse, just different. Your average one part story has about 30-90 seconds of setup and character development before the actual plot kicks in, with lengthy scenes of running around dingy corridors to maintain the illusion things are happening. The action continues until literally seconds before the end of the episode, at which point the Doctor simply darts offscreen and it's ON THE NEXT EPISODE OF DOCTOR WHO Z...
God help you if you're watching a two parter, for that matter. Due to the mysterious rule instituted by Moffat that all part twos should start in a different place and, if possible, different time from the previous episode, cliffhangers end up with dismal resolutions and stories often feel like 45 minutes of a first act and 45 minutes of a second and third act, or vice versa. In Moffat's defense, though, when the series has tried the classic who cliffhanger structure, it hasn't gone well.