As someone who reads the manga. It did not have flashbacks. They were put in to increase episode time and decrease source content usage to distance the anime from the manga. One Piece needs a hiatus badly. The anime that is.
The problem is that the pacing of the manga is just fine, but the anime just drags on and on. They animate only one chapter per episode (which is not much for a weakly manga, it's usually 2-3 chapters for other animes based on weakly mangas) and sometimes even less and they fill the rest of the episode with reactions and flashbacks. This is done to keep the manga ahead of the anime but it made the anime feel really slow
Every boss in every anime/movie/game: I`ll wait for the main protagonist to come to me or I`ll send increasingly stronger waves of my minions against him always failing short of defeating him.
Assemble the fleet, we give chase!
*how many ships, mum?*
ALL OF THEM!
*how many soldiers mum?*
*all your sons and daughters too?*
call Kaido too