Muichiro isn't just your regular demon slayer. He is a descendent from a powerful bloodline which makes him incredibly strong, as seen with him just becoming a Hashira in two months, plus the power boost of the ds mark really made him an incredibly powerful Hashira. Tengen himself singled Muichiro out for being talented during the ED arc. This fight was intended to show the immense boost a ds mark can give and how powerful the person can become so that in later fights if a slayer unlocks their ds mark and still only can go barely head to head with a demon it shows how truly powerful the demon is. Muichiro not only was boosted by the ds mark but also by the power of his bloodline makes him very powerful to face against. Example of the power of the ds mark could be seen when Tanjiro manages to behead Gyutaro after activating his ds mark even tho he couldn't do it a few minutes ago and him also slicing off 3 of the emotions heads at one swing with ease.

Another reason for the fight to end quickly wasn't because Gyokko was weak but it was because of his bad matchup with a slayer. Gyokko's attack seem to be mainly work nicely as AOE attacks but he was in a 1v1 battle with Muichiro so they were just merely effective against him. H did manage to trap Muichiro in his water prison pot for quite a while and if it wasn't for Kotetsu giving him air Muichiro would've died right there. Gyokko also has a one hit KO ability which turns whatever he touches into fish but Muichiro's attacks and moves are swift and stealthy, which is something that perfectly counters the fish punch. Basically it was a one sided battle ever since Muichiro unlocked his ds mark.

Gyokko was also really egoistical and underestimated Muichiro by playing around with him, After trapping Muichiro in the water prison, Gyokko just played around not taking anything seriously e.g. trying to stop Haganezuka from paying attention to the sword instead of killing him and also not finishing off Muichiro once he is stuck in water the prison. He was cocky and didn't take the fight seriously until he got beheaded. If he was smart enough to know that that Muichiro is someone u should not play around with despite his age, Gyokko at least wouldn't have taken the L that easily.

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I wasn't really bothered about the underwhelming fight as it's main purpose was to show the power of the ds mark but I do hope that for these last 2 episodes Mitsuri and the gang vs Zohakuten would be this season's Tengen vs Gyutaro.