Late running and time saving

Well, my first question is, was the train (semi-)fast to begin with? Did it then become (semi-)fast, or was it literally just Wembley Park that was being missed out.

In any case, Wembley Park is a fairly busy station and line speeds through there are fast enough that it probably saves a little bit and that could make all the difference at Watford South Junction or Chalfont & Latimer.

I think it's fifty, too, though it's line speed before and after the station. Not sure what line speed is, there, though! 60?

Assuming we're right, clearly, slowing from 50, and then accelerating back to 50, is gonna take a fair amount of time - more than if the speed were, say, 30 or 35, as it might be in central London. And then, of course, stopping at a station and serving it is a much bigger time penalty when you could've gone whizzing through at 50 than it would be if you could only have gone whizzing through at 30. And remember, you can pass through the platform at full speed up at Wembley Park - not something you can usually do when skipping a station, you usually have to slow to 5 mph on the approach to the starter.