I think the direction and animation will make up for any slow pacing in 913.
Toei invited talented animators that worked on things like Attack on Titan sesaon 1, OPM season 1 and the fate series to work on the Luffy/Kaido fight. So 914 should be a spectacle similar to 870.
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Any extra time we spend with WB is a bonus for me. I'm hyped for the onigashima raid after the flashback, but if we get more WB, Roger, Toki, Laugh Tale, Void century info I don't mind the flashback extending a bit more.