To sum it up, it is possible for Zenitsu to fight without fainting and it has been shown clearly in the manga. While he only perfected the first form, he trained hard enough to create a new form that didn't exist before.
His real power shows when he is aware and awake, he was able to outmatch the new upper rank six demon, Kaigaku. At this point Zenitsu has manifested his speed and power, and he was also able to keep up with the Demon King, Muzan, even when weakened.
Apparently Zenitsu couldnt use anything else besides the first form of Lightning breath. Later in the manga, it was revealed that he was able to create his own breath technique of Lightning. And he's able to fight full strength without sleeping ..
Zenitsu has some combination of RBD, narcolepsy, sleepwalking, and lucid dreaming that allows him to behave as if he were awake while asleep. Thanks to his extensive training and superhuman hearing, he's able to detect and fight demons in this state.
Through sheer will and anger, Zenitsu was able to overcome his cowardice and stay awake in battle against demons, shown during his bout with Kaigaku and Muzan, especially in the fight with the latter in which he refused to give up and persisted despite being enervated and injured.
While unconscious, Zenitsu is a force to be reckoned with, but his actual power is discovered when he is awake. While awake, Zenitsu was able to match the upper rank six, Kaigaku.
Tanjiro Is Strong, and Stronger than Zenitsu as he uses Sun Breathing.
Hence, Zenitsu is stronger than Inosuke in Demon Slayer.
Zenitsu Agatsuma is a part of the Demon Slayer Corps who the previous Thunder Hashira trained. Zenitsu is a skilled and able member of the organization but gets easily scared when facing demons. But as the series continues, we watch him grow as he eventually overcomes his fear of demons and fights them head-on.
How fast is Zenitsu?? I mean it's difficult to tell, but at least 750 mph since he can break the sound barrier.
The reason for this is pretty simple. Zenitsu wasn't able to master the other 5 forms. He never wanted to learn any breathing techniques. or fighting techniques, nor he wanted to join the Demon Slayer Corps.
A Thunder Breathing user that reaches the rank of Hashira is dubbed the Rumble Hashira ( 鳴 なり 柱 ばしら , Nari Bashira?). In the manga, whenever a Thunder Breathing user performs their techniques, they visualize themselves creating lightning that is bright yellow in color with a white outline.
After mastering his Thunder Breathing, Zenitsu would eventually become capable of awakening his Demon Slayer Mark. As this mark would enhance his strength and speed, he can overwhelm Gyomei to a certain extent, but he still won't manage to defeat him.
Thunder Clap and Flash—Godspeed is the most powerful of Zenitsu's derivations of the Thunder Breathing first-form technique.
Zenitsu Agatsuma: The Sense of Sound
He can hear movements from humans, animals or demons, and if he truly concentrates he is able to read people's thoughts. This ability helps him locate demons who may be deeply hidden, sensing danger from miles away.
One major part of why Zenitsu is scared of demons has to do with his Breathing Style Form mastery. Zenitsu uses the Thunder Breathing Style, but has only mastered the First Form for combat use.
Zenitsu — Anxiety
To the surprise of absolutely nobody, Zenitsu is anxiety incarnate. He starts out the series so paralyzed by fear that he can't even fight when he's conscious, and so he's only able to fight when he's passed out.
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.
2 Zenitsu's Whining Is Irritating
Zenitu's incessant whining becomes rather annoying. Although there are times Zenitsu's remarks can be humorous and entertaining, the excessive complaining quickly becomes bothersome. Zenitsu's puerile decision-making before battle makes him a liability.
But, comparing the two characters in their best forms, Zenitsu might be just a tad bit faster owing to his Godspeed move.
slow ?! , Deku was able to dodge and hit in a split second. And yet he is clearly faster than Zenitsu .
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.
1 Sun Breathing
It is a legendary breathing style that mimics the sun itself and incorporates that into the user's swordsmanship. Against the Demons, this is known to be the most effective breathing style as it is extremely versatile.
Hinokami Kagura, also known as Sun Breathing, is easily one of the strongest Breathing Styles in the series by a wide margin.
Sun Breathing has the second highest number of known techniques of all the breathing styles. It has twelve forms that can all be combined into an extremely powerful thirteenth form. It is second only to Moon Breathing which has sixteen techniques, employed by the Upper Rank One demon Kokushibo.