Sorry, if you think the situation for Enies Lobby and Alabasta was the same as the War at Marineford, you must be watching/reading a different anime/manga than me.

Enies Lobby they broke into small groups in order to search for Keys, and to find Robin. CP9 split up to force the Strawhats to split up to slow them down. When the group splits up into pairs or fewer, you get duels. Alabasta had other reasons that dictated them splitting up.

There was no reason for people to split up like this at Marineford. Instead, we get a frantic free-for-all. And no, the entire battlefield isn't focusing everyone on Luffy. You don't constantly see all 3 Admirals, all the present Shichibukai, and all the Vice Admirals breathing down Luffy's throat. The story follows Luffy through the battle, but the story isn't telling all of the battle.

Like for example, Luffy is running through the battle, and Smoker appears. Its time for a duel right??? Oh wait, no, this is a huge all out free-for-all war, with tons of different characters running around fighting whoever they want, and Hancock comes and attacks Smoker. The story moves on with Luffy, and doesn't even focus on Smoker vs Hancock. Same with stuff like Crocodile vs Don Flamingo.

So, we see this happening to Luffy alot, that people will intercept his enemies, and we accept these as valid reasons as to why Luffy isn't having a hundred different all out duels. Why then do you assume that the people who intercept those enemies immediately abandon their fight, or abandon their fight for no reason? Any number of things could happen off screen, but you assume that "If it isn't shown, nothing happens"? Their own fights could have gotten interrupted for any number of reasons, one person could have retreated, one person could have been temporarily incapacitated, etc etc.

And yes, it is that the anime pacing isn't what it should be. Don't try telling me that all those Hancock-fantasizing-about-Luffy jokes, or Buggy-being-Buggy jokes do anything other than mess up the pacing.