Fyi, it's actually easier for trains to be on time the longer the journey is - that's because sane travel times aren't calculated at max speeds. So if there is any delay, you can go at a higher speed than what was used in the calculation to catch up. On shorter journeys there are simply fewer opportunities to catch up.

Now, I wouldn't be exactly shocked if DB using too high speed assumptions in their stated travel times was part of their problem with delays.