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.
However, the thirteenth form is the one that would land the final blow on Muzan. This form is a repetition of every Sun Breathing form performed in succession that would kill Muzan by hurting all of his vital organs.
At best it's through scaling but that still doesn't prove anything since you can't confirm how much faster. Yes, Tanjiro is speed of sound at the start of the series but light is roughly a million times faster.
Tanjirō's resemblance to Yoriichi (like from Muzan's perspective), would trigger his memories and cause him to accept death. AND since its against Kokushibō and not Muzan, there's not poison that'll mess him up. So I would say Tanjiro would win.
Given how talented Tanjiro is, it's likely he'd defeat Zenitsu, even if the latter could fall asleep and call on his powers. Tanjiro's demonstrated his ability to take down more powerful demons and adapt to training on more than one occasion, so it just makes sense.
1) Inosuke Hashibira
Although Inosuke possesses immense strength and endurance, he's no match for Zenitsu's insane speed and his Thunder Breathing.
Fortunately, Zenitsu was able to single handedly beat the Upper Moon 6. Not only did he defeat the demon, he also created a seventh form called the Flaming Thunder God. These are the demons who have been given the title of Upper Moon 6 in Demon Slayer.
But if we are talking about the human form, then it's a big fat NO, because Tanjiro has just mastered the Sun Breathing whereas Yoriichi is the inventor of it, also he's way more powerful and holds an unfathomable battle intellect, larger than any Demon Slayer in history, so in conclusion, no one is remotely close to ...
Mitsuri Kanroji is another Hashira who Tanjiro could definitely beat in Demon Slayer. Mitsuri's main trick up her sleeve is her muscle composition, which makes her muscles eight times denser than that of a normal human's. In other words, she's incredibly strong and powerful, and tough with plenty of endurance as well.
So in the end DEMON KING TANJIRO IS A WHOLE LOT MORE POWERFUL THAN YORIICHI. Also not to mention that Tanjiro is literally invincible I mean to be honest Tanjiro could beat anyone in all of anime and in other series bc of his IMMENSE power.
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.
Zenitsu Agatsuma is a member of the Demon Slayer Corps who is a wielder of the Thunder Breathing Style. While he is ridiculously fast for someone who's human, he isn't necessarily the strongest character in the series.
1/10 Tengen Uzui Makes Sound Look Slow
Tengen Uzui may or may not be the most powerful Hashira overall, but he is certainly the quickest of them, and even the Upper Moons would be hard-pressed to match his movements in battle. When Tengen gets serious, he can seemingly teleport, arriving at his destination in a flash.
Through his acrobatic skills, swordsmanship prowess and sense of smell, Tanjiro managed to successfully decapitate the demon. Tanjiro's extraordinary combat prowess and superhuman physical attributes allowing him to overpower Rui, a member of the Twelve Kizuki.
Twelfth Form: Flame Dance
The sheer power of this attack makes it a formidable move. The Flame Dance involves a combination of both a powerful vertical and horizontal slash. If used properly, it can spell doom for most foes.
Thirteenth Form: Dance of the Fire God?) - The user continuously performs all twelve forms of the breathing style in repetitive succession to increase the accuracy and agility of his movements while reducing fatigue.
Giyu Tomioka and Sanemi Shinazugawa attend as the final surviving members of the Hashira, and together they they get a confirmation that not only are the demons all eradicated, but that the Demon Slayer Corps will be disbanding.
5) Yoriichi
Case in point: he was the only one who could single-handedly take on Muzan and drive him to a corner. Yoriichi would be able to defeat every single Hashira on this list without breaking a sweat. Such was his reputation and skill in swordsmanship.
As it turns out, Tanjiro has received a few promotions during his service. However, saying that Tanjiro has become a Kanoe doesn't mean much on its own. The Corps' ranks and functions are fairly straightforward.
It was Yoriichi's sword, he found it inside Yoriichi Type Zero doll when he destroy the head. And then hotaru took the sword to fix it. Tanjiro also use Kyojuro's tsuba in this sword.
Tanjiro Is Strong, and Stronger than Zenitsu as he uses Sun Breathing. BUT! Tanjiro isn't Faster Than Zenitsu, not at all. He is Strong but Slow.
1/10 Tenth Form: Constant Flux/The Dragon of Change
The most powerful water move in Tanjiro's current skill set, this is a form that gets more and more powerful with each rotation that he puts in. It's number one on the list because it has, theoretically, no ceiling when it comes to its intensity.
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.
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.
Yoriichi was born with the Demon Slayer mark and prowess in swordsmanship. His powers were beyond comparison as he defeated Muzan once and had him on the verge of death until he managed to escape.