The explanation I was given as to why the District (and Circle/H&C) is
sometimes driven at such a dull pace is because there's so much slack
in the timetable, the only gain from rushing means catching the next
train up, especially in town. It's better to cruise along at a merely
speed rather than stop-start-stop-start at every signal.

Despite that the trains are almost 30-35 years old, they can still be
very nippy when they want - especially on the long open sections to
Upminster/Wimbledon/Richmond.