Undoubtedly, Muzan is the fastest and strongest demon in Demon Slayer.
Among the current generation members, Shinobu and Zenitsu are the fastest characters in the series. According to the Upper Moon 2, Shinobu was the fastest opponent he had faced and was unable to react to her attacks due to its speed.
Then again, the good news is that Hulu Japan posted an official number regarding Zenitsu's speed whenever he is performing Thunderclap and Flash. Basically, the post says that he is as fast as 172.1 km/h whenever he performs his Thunderclap and Flash move.
Sound Hashira
He is the fastest Hashira, and the second physically strongest. Obviously, then, Tengen Uzui is rather proud. He is demanding, fussy, boastful, and prone to selfishness. But he is also ultimately kind and caring, with great love for his three wives as well as the other Hashira.
That would be 102,316,500 miles per second, or 228875492 mph. That's roughly 300,756 times faster than the speed of sound.
10) Zenitsu
He certainly has speed and was able to keep up with Daki during the Entertainment District arc. While some might say that he could one-shot the Upper Moon demon Kaigaku, the demon was nowhere near the level of an Upper Moon since he was not used to his powers.
1) Inosuke Hashibira
Although Inosuke possesses immense strength and endurance, he's no match for Zenitsu's insane speed and his Thunder Breathing.
2) Sanemi Shinazugawa
His fast movement, combined with his excellent swordsmanship, makes him a remarkable demon slayer. Most of his attacks are purely offensive due to his breathing style. Being a user of wind breathing, he is undoubtedly fast.
172.1 Km/h Whenever he performs his thunderclap And flash move.
Tanjiro isn't Faster Than Zenitsu, not at all. He is Strong but Slow. So, Zenitsu Will Probably Chop Him Down to bits before tanjiro even pulls out his sword properly.
With all of that, what Zenitsu's true power is his spirit. Zenitsu might be a coward at times, flee from danger instead of facing it head-on, but when he's needed, he'll fight for his friends.
As you've already said, Zenitsu's thunder breathing would make his Breathing Form attacks quicker but if you're talking about nimbleness and agility in battle then Shinobu is light years faster than Zsnitsu.
Godspeed ( 神 しん 速 そく , Shinsoku?) - An extension of the Thunderclap and Flash technique created by Zenitsu. The user performs Thunderclap and Flash while greatly augmenting the speed of the technique.
How fast Muzan is? That would be 102,316,500 miles per second, or 228875492 mph.
The users of this particular breathing style are blisteringly fast, and Zenitsu is someone who was trained by a former Thunder Hashira. Zenitsu's speed shocked his senior, who too was a wielder of Thunder Breathing Style. He turned into a demon and his physical abilities were enhanced drastically.
Killua Zoldyck's "Godspeed" allows him to reach a velocity comparable to Zenitsu's own. He has many advantages that would allow him to dominate his demon-slaying counterpart, particularly through differentials of experience.
Agatsuma Zenitsu is a mini-boss in Demonfall that uses Thunder Breathing. Demons can get a quest for executing him, however Slayers can still kill him and get exp. This mini-boss has 350 HP and gives 300 XP upon execution, and respawns fast just like the Green Demon and Axe Demon.
Zenitsu is at least close to Hashira level by the end and Godspeed has limited uses because it disables his legs after those uses. Zenitsu's faster but if this was an actual race or marathon, Shinobu would beat him.
Thunder Breathing has 6 standard forms in total, with the first form said to be the foundation for all the other forms. However, Zenitsu Agatsuma developed a Seventh Form in order to stand on equal ground with Kaigaku, his former senior disciple under Jigoro who had become the Upper Rank Six of the Twelve Kizuki.
1) Gyomei. Gyomei Himejima is the strongest Hashira in Demon Slayer. This fact has been established in the manga as Tanjiro, Inosuke and Zenitsu all agree that he is the strongest member of the Demon Slayer Corps.
Mitsuri. The love Hashira and upcoming featured character of season 3, Mitsuri is best known for her extreme affection for everyone in the Demon Slayer Corps.
7/13 Kyojuro Rengoku, The Flame Hashira.
3) Zenitsu Agatsuma
Zenitsu, with his insane strength and lightning speed, can eliminate every Upper-Rank Demon in 1v1 except Upper Moon 1, and Muzan because in terms of strength they are more formidable than him.
When facing Kaigaku during the Infinity Castle arc, Zenitsu debuts his brand new exclusive Seventh Form to Thunder Breathing. With this, Zenitsu finally has more than one tool in his arsenal and has overcome his own personal doubts and fears.
Although Zenitsu managed to overcome Rui's elder brother, it's unlikely he'd be able to do the same with Rui. Given that Rui is one of the Twelve Demon Moons, he's an incredibly powerful creature — one that almost kills Tanjiro, who's more capable than Zenitsu is when it comes to dispatching demons.