Tanjiro Is Strong, and Stronger than Zenitsu as he uses Sun Breathing. BUT! Tanjiro isn't Faster Than Zenitsu, not at all.
Tanjiro's confidence alone would allow him to overpower the other boy, but his knack for strategizing and skill with a sword would certainly help too. Given how talented Tanjiro is, it's likely he'd defeat Zenitsu, even if the latter could fall asleep and call on his powers.
Demon Slayers are known for their strength and bravery in battle, but not Zenitsu. Despite his cowardice, he's one of the strongest in the series.
But, by the final chapter, there is a clear difference between the two in terms of combat ability as well as speed and strength. Hence, Zenitsu is stronger than Inosuke in Demon Slayer.
1) Inosuke Hashibira
Although Inosuke possesses immense strength and endurance, he's no match for Zenitsu's insane speed and his Thunder Breathing.
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.
Unfortunately, the answer to this question would be, no, he did not. Zenitsu, along with Tanjiro and Inosuke got way stronger than average corps members with training. In fact, he became one of the very few to even invent a new form of Thunder Breathing. However, his dream of becoming a Hashira was never realized.
Zenitsu's rankings in the popularity polls are as follows: Zenitsu ranked 2nd place in the first popularity poll with 4,299 votes. Zenitsu ranked 1st place in the second popularity poll with 17,451 votes. Zenitsu also appeared in The Promised Neverland's 2020 popularity poll, ranking 29th place with 44 votes.
Nezuko Kamado
Zenitsu also protects Nezuko multiple times when she is in danger, evident during Zenitsu's first meeting with Inosuke Hashibira and the battle against Enmu in the train. Eventually Zenitsu and Nezuko would marry and start a family as evidenced by their descendants.
This might be a little difficult to digest, but Zenitsu is not one of the fastest characters in Demon Slayer. He certainly has speed and was able to keep up with Daki during the Entertainment District arc.
Clearly, Tanjiro is much stronger than Zenitsu in Demon Slayer. Tanjiro beats Zenitsu in every relevant category to the question, whether fans directly see this or it's implied. The unique powers of Demon Slayer would normally make it difficult to power scale and compare two characters.
Exploring Zenitsu's fighting abilities in Demon Slayer
Zenitsu has perfected the first form only and is seen performing it only when he's unconscious. But the manga shows that he can fight without fainting or going unconscious.
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.
He's mastered the first form to utter perfection making it deadly and he even made a godspeed version. I don't think he could take Kokushibo, but with enough training and a mark he would obliterate the other upper moons. Doma go bye bye.
Going beyond this, Giyu is one of the most combat proficient Demon Slayers seen throughout the entire series. Furthermore, his strength and abilities are always portrayed as seemingly a cut above the rest of the Hashira, solidifying his expected victory against Tanjiro.
Zenitsu Agatsuma vs Kaigaku is a battle that took place in the Infinity Castle. It focuses on the battle between Zenitsu Agatsuma against Upper Rank Six Kaigaku.
She was ranked in 3rd place in the first Demon Slayer character popularity poll with 3,319 votes. IGN regarded Nezuko as one of the series's most popular characters and noted how her design elaborates the way she sees humans as good people as a result of hypnosis.
That being said, Tanjiro's rank is unknown, and his rank was last revealed in the Entertainment District arc. In that arc, they were at Kanoe, which is fourth from the bottom. Since Kiriya Ubuyashiki announced the disbanding of the organization, Tanjiro doesn't get the chance to become a Hashira.
As a result, Tengen Uzui has to deal with the moniker of being the least powerful Hashira around.
Yes, he faced one of the requirements to become one!!! He defeated on of the 12 Kizukis by himself and is at the rank of Kinoe! But, of course, the Demons Slayer Corps disbanded, so I guess he doesn't have a chance to be one, but either way I'd say yes!
When it comes to pure strength, Shinobu Kocho is the weakest Hashira in Demon Slayer. She doesn't even have enough strength to decapitate a demon's head and she makes up for it by injecting poison into the demons.
While awake, his nerves and fear cause him to freeze up so he cannot move. When his life is in danger, his nerves and fear grow so strong that he passes out. But while asleep, his nerves are no longer holding him back, allowing him to fight and move with great judgement, precision and skill.
Zenitsu has such a wide range of emotions, all of which have their extremes. It's part of what helps make Zenitsu such a likable and memorable character, as well as making him really funny in the process.