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.
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.
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.
Final Thoughts
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.
Inosuke always displayed raw strength and courage from the first few chapters of Demon Slayer. One might even argue that he was far stronger than Zenitsu during the first arc.
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.
Overall, Zenitsu is an extraordinary powerful Demon Slayer capable of singlehandedly bringing down an Upper Rank, which said to be able to overpower 5 Hashira, in 10 minutes, albeit he would have beheaded Kaigaku in a heartbeat if it wasn't for his feelings of melancholy and pity.
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.
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.
Unconscious Combat: Zenitsu has the ability to fight when he sleeps, which allows him to awaken his true strength. 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.
Why Zenitsu could not become a Hashira in Demon Slayer. Fans have wondered whether Zenitsu ever got to realize his dream of becoming a Hashira at all. 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.
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.
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!
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.
So in a nutshell, Zenitsu wasn't able to master the other five forms, but later in the Infinity Castle Arc, during the battle with Kaigaku, Upper-Rank Six Demon, he created a Seventh Form: Flaming Thunder God.
Zenitsu may be faint-hearted and tend to pass out when he gets too afraid - thus placing him into a bizarre kind of sleep - but he actually has stellar Nichirin Blade skills and breathing techniques. Fortunately for him, being unconscious is the key to activating his combat skills.
But that's also highlighted a major weakness of Tanjiro as he's so honest that he can't even tell a lie without his face contorting in a highly unnatural way. It's almost as if his body repels the fact that he's trying to lie, and his pure and honest soul just won't let him do that.
Tanjiro is certainly strong in his own right, and he's strong enough to defeat many of the characters in the series, whether they be demon or Demon Slayer. But he's also basically a newbie within the Demon Slayer Corps, so there are still plenty of characters still able to defeat him.
7/15 Avatar Aang Outclasses Tanjiro In Sheer Power (Avatar: The Last Airbender) Aang would get along great with Tanjiro if they ever met since they prefer peace over the battle and want to protect the innocent. They both think people should get a second chance. However, Aang certainly outclasses Tanjiro in sheer power.
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.
The spider demon explains that the spider that bit Zenitsu earlier was one of his, and Zenitsu has been poisoned and will slowly start turning into a spider. As always, Zenitsu goes into a state of frenzy, and then he climbs a nearby tree.
After the deaths of Daki and Gyutaro, Kaigaku was promoted to become Upper Moon Six of the Twelve Demon Moons due to his power and confronted Zenitsu as a rematch he always desired.
13/15 Zenitsu Agatsuma Defeated An Upper Moon On His Own
He falls asleep when he crumbles under the weight of his own fear, and his body fights on instinct alone, utilizing his Thunder Breathing Style. He proved that he can be a capable fighter even while awake, defeating Upper Moon 6 on his own.
3) Zenitsu Agatsuma
If he could, Rengoku would win simply because Zenitsu cannot perform Godspeed more than twice. Since Zenitsu only knows the first form, Rengoku would know what to expect and accordingly counter his moves, taking him down in the end.
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.