I am sure most of the fans of One Piece agree with me on this one. The entire story of Dressrosa is progressing slowly. If you follow the anime like me, rather than the manga it's a very painful wait for something to happen.
Is there any particular reason behind it? Do the decision makers want One Piece to go on another 15 years. Some facts I know which might answer this are:
Previously one episode was made up of 2-4 chapters of manga, now its 1-2 chapters per anime episode.
Character Introduction: The amount of character introduction in this arc is too much which I think we can safely assume that they have some purpose in the upcoming arcs. (Similar to how so many pirates were supporting Whitebeard in the war)
⚠ The answer below contains spoilers. Read at your own risk.
As of this writing, the 10 longest arcs with their respective number of episodes are:
Clearly, Dressrosa is roughly double the length of the second longest arc until now.
The major reasons behind this are:
As you point out, the number of characters introduced during this arc are far too many as compared to any other arc. To list the major ones:
Bellamy is revealed to be alive and has visited Skypiea
Buggy is confirmed to have joined the Shichibukai during the time skip
Colloseum Fighters (Mainly Rebecca and Kyros)
Kaido (seen for the first time, as opposed to the silhouette we have been seeing)
Kuzan is revealed to have become associated with the Blackbeard Pirates
Revolutanaries such as Sabo, Koala and Hack
Rob Lucci, Hattori, and Spandam have joined CP-0
Many characters make their first appearances after the time skip. The characters (in the order they were reintroduced) are: Jewelry Bonney, Brannew, Sakazuki, Bellamy, Jesus Burgess, Bastille, Sabo, Koala, the Gorosei, Urouge, Capone Bege, X Drake, Sengoku, Tsuru, Rob Lucci, Spandam, and Hattori.Source
Plot development: A lot of plot was covered in this arc, from introducing new story lines to filling in the gap that was left after the timeskip. To list a few of the things coveredSource:
Law's backstory and history with the Donquixote Pirates is shown.
Luffy activates a new technique called Gear Fourth.
Luffy fights and defeats Doflamingo with the other crew members having their own fighting scenes with their respective opponents.
More information about Haki is revealed during this arc. According to Law and Luffy, if Haki is overused, it will not be able to work for a certain period of time until it recharges.
The gladiators and dwarves who allied with Luffy offer him an alliance with 7 representatives of each crew, giving a total of about 5,640 pirates. He refuses the offer but they choose to pledge their allegiance to him even if he refused, forming the Straw Hat Grand Fleet.
Trafalgar Law is revealed to have the "D." in his name. It is also revealed that anybody that is a part of the Will of the D. is considered "an enemy of God".
Slow Pacing: As you pointed out, the pacing of One Piece has been slow during this arc. As mentioned in this article:
TOEI has decided to translate 1 manga chapter to 1 anime episode. Which is something that doesn’t really always work since narrative in comics and audiovisual pieces is not the same. We’ve also seen too many unnecessary reaction shots. For example, recently we got nearly a quarter of an episode of just toys transforming back to humans. Or hundreds of panning shots of Sabo (without showing his actual face until last week’s episode). Or repeated flashbacks, like Rebecca and her father.
This is happening because the manga and the anime are pretty close at the moment. Usually, an anime makes fillers in cases such as this. However, One Piece fillers are rare, and the one that we did get prior to the Dressrosa Arc: Caesar’s Retrieval Arc, was only 3 episodes long. We also got the 3D2Y Special, but it still doesn’t seem to be enough.Source