The situation is basically same wit Arlong and Moria.
Both lost to guys who were Admiral/Yonko lvl.
Both were weak(in terms of post TS power level). Nothing suggest they lost there strength after their clash with an admiral/yonko then why are people so adamant on a fact that Moria which fought Kaido was way stronger then the one who faced Luffy?
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Time in the East Blue is close to impossible to map out. Grand Line is a bit easier though. Loguetown, Reverse Mountain, and Whiskey Peak are the same day. They leave Whiskey Peak the 2nd day. Then several days pass when they reach Little Garden so I'm guessing it's been a week now. They are attacked by Wapol on the 8th day and reach Drum on the 9th. Four days pass and they meet Mr. 2. Five days til they get to Alabasta. The war breaks out two days later. Luffy sleeps for 3 days. They leave the next day, then get to Jaya the same day. They get to Skypiea the next morning and defeat Enel the day after that. So it's been like a month at this point. Then they reach Longring Longland and stay for four days. Three days later they reach Water 7. The next day is the Aqua Laguna. That night they raid Enies Lobby up until the next day. Luffy rests for two days when Garp shows up. Sunny is built in three days and they leave. Several days later, they reach Thriller Bark. They defeat Moriah at sunrise the next day. They sleep for one whole day when Brook joins. They leave two days later. So all this is between three and four weeks. Several more days pass when they arrive at Sabaody. Luffy is sent flying by Kuma and reaches Amazon Lily the next day. He wakes up from the mushroom after a day and befriends Hancock. Five days later they reach Impel Down. The next day is the war. Two weeks pass when Luffy wakes up at Marineford.
So about a month has passed since they left Thriller Bark when Luffy decides to train for 2 years. Post-timeskip, it's only been three days. They reach Fishman Island and stay for a day. They leave and go to Punk Hazard. The third day is now on Dressrosa.
^That is a rough calculation:
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So he did get weak after all? ; )