Pokémon isn't exactly known for its laser-focused plots, but one Reddit user posits that all of the early seasons' filler episodes might be Brock's fault. This actually isn't too Farfetch'd -- the show became much more focused on character development after Brock left the group and, as a result, could go into larger, overarching plots unrelated to each region's Legendaries.
The original poster admits that out of universe, the reason for the tighter-plotted journey after Brock's departure is likely due to the sped-up release dates of the games, going from a new generation every four years between 2002 and 2010, to a new generation every three years. But, despite real-life factors, for an in-universe explanation -- from Brock's delayed start, lack of directional skills to his lovestruck nature -- they make a convincing argument.
The Redditor also notes that a recurring joke from earlier in the series is Brock's lack of directional skills, which, when coupled with his position as the group's "big brother," often meant he was leading the way into another "lost" episode. Even when they do end up on the right path, it's usually to a Pokémon Center which, as the theorist suggests, would give him as many chances with his beloved Nurse Joy as possible.
However, according to the Redditor, Brock's lack of directional skills are the result of caring for all his brothers and sisters at the Pewter gym. Brock didn't have time to learn many skills that trainers would normally use on their journey -- like how to read a map. While no doubt Ash and co. are thankful for his culinary skills, it doesn't help them get from point A to B. They also bring up the fact that Ash's companions in Unova, Kalos and Alola are all much better at showing Ash around, whether its due to familiarity with the region or just having a better GPS.
But that doesn't mean the theory is without its holes. While they note that, in Hoenn, Max is a kid liable to make mistakes with the PokéNav, the fact is that Max was the person directing the group for that section of the journey. Furthermore, Max, having learned Misty's ear-pull trick, was more than capable of keeping Brock in line and on-track. He was also precocious and consistently shown to be one of the smarter characters, knowing type advantages and which moves would increase stats -- things Pokémon-loving Ash had no clue about when he started his journey. Other the PokéNav's unreliability at the hands of the writers, there's no doubt that a kid as smart as Max knew where he was going, even if Brock didn't -- and yet the filler continued.
But there is one other reason why Brock might've forced the group to beeline for Pokémon Centers. While initially wanting to be a Pokémon Breeder, Brock's arc sees him question that goal, eventually discovering that he actually wants to be a Pokémon Doctor. It could be that, even from the start of his journey, Brock was drawn to the Centers for what Nurse Joy represented. Compassion, kindness... and medical expertise. His desire to be a doctor could easily have grown out of admiring and watching Nurse Joy's work, so in that way, any episode with Brock in a Pokémon Center wasn't filler at all.
Regardless, the fact is that every Pokémon journey is different, and Brock's was a slow and steady trek as he began to understand his passions and who he wanted to be. Is Ash progressing faster now that he's gone? Debatable. But, one thing is for sure: Ash spends way less time in Pokémon Centers today than he did when he traveled with Brock.