When comparing a vast majority of older anime to series from the last decade, the pacing gradually becomes quicker over time. Though it went unnoticed back in the early 2000s, popular series like Naruto, Bleach and Death Note thrived despite their slow-paced narratives. In each of these series, all the characters and lore are given the utmost explanations and development so viewers would carry that information with them throughout the story.
Those series also tended to have a larger amount of details laid out to support open worlds and longer series -- and seasons for that matter. If they were debuting today, they likely wouldn't stand out as much as other series that reach their peaks far sooner. Granted, there is still merit to slower pacing today, as anime like Vinland Saga and My Hero Academia are praised for their stories despite slower pacing, continuing the old-school trend.
All things considered, a slower pace in narrative does mean more of a build-up for characters and lore. This certainly helps set up for bigger plot points later on, and includes added closure and character growth to enhance audience payoff. In short, the more details given in a story could in turn provide more for viewers to look forward to. Much of this slower pacing was once a part of the setup, practically being standard practice and therefore accepted by fans. Fast-forward to today, however, and the once-beloved minute details that framed an anime are now cut in exchange for quicker engagement.