the swamp demon when it has no fte feats
He has by virtue of fighting Tanjiro and Nezuko and being somewhat comparable (despite being slower by an unspecified degree). By this logic, a characters who fights against MHS+ and FTL characters is actually below subsonic unless they are portrayed to move FTE for the audience.
and other weaker demons like the temple demon are never portrayed as fte in the manga.
The anime is secondary canon as it is overseen by the author (Gotōge-san is involved in checking over the scenarios, character designs, settings, and overall they are intimately involved in the anime adaptation process. I was in constant contact with Gotōge-san throughout production. They actually have a great deal of trust in ufotable, so it was less about requesting changes and more about giving them useful advice to make the best work possible. In my opinion, it was a great working relationship.). Also, weaker newly created demons are indeed portrayed to move FTE in the manga.
Kanroji who scales directly to him is hit with a sound attack before she can stop him
Then this is either a low outlier we can ignore, or it is something that can explained in-series by the character being caught off-guard by an invisible AoE attack. This can be explained by the later, therefore we shouldn't assume an outlier or that the scene is 'completely and irreconcilably inconsistent'; Mitsuri was unaware of Zou Hakuten's fullset of abilities, and was planning to behead him and retreat as soon as she lands on the ground; she beheaded him but an unexpected invisible AoE attack hit her before she lands on the ground.
and is later stopped by Nakime's ability twice
SFX are in-line with
The Coconut Effect; we know Dragon Ball characters fight at FTL, but if they are portrayed to do so without sound effects it would annoying and immersion-breaking for the audience (regardless if it is unrealistic), so this does not constitute a serious argument. Nakime's ability for Spatial Manipulation is likened to someone moving their own body.
Sanemi also attempts to use a gun on Kokushibo and expected it to work.
By a Nichirin shotgun, which means that it can be enhanced for a variety of effects when combined with Breathing Techniques. Massively slower demons are able to react to a shot from stealth (and Tanjiro said that they allowed themselves to be hit and decapitated on purpose), and even block them. When Genya tried to snipe Kokushibo from stealth, Kokushibo casually blitzes him after he fires, even though Genya was able to blitz the demon who blocked the pellets. If Sanemi was expecting the Nichirin pellets to hit Kokushibo then he was he further enhancing their speed just like he enhanced the gun's durability to block Kokushibo's slash (he wasn't expecting them to kill, only distract him enough that he can free his sword)
There's a calc for Muichiro that produced MHS results. However when I consulted Dargoo Faust he said he disagreed and sees no reason for the slashes to be limited to one per fish and that the feat could be performed with far less slashes.
Um... One slash per fish is literally the highest possible end for the feat and it produces MHS+ results. The one that got accepted is slashing 10 fish on average per slash since Muichiro was only aiming directly downwards.
To gather low-level FTE and supersonic feats that scale to BoS Tanjiro and therefore superior characters:
- Temari Demon and Arrow Demon moving FTE
- To anticipate the objection of "why doesn't he rush the Drum Demon in a straight line if he is faster than sound" (which Tanjiro said he might have been able to do if he wasn't injured), this isn't an effective tactic since the Drum Demon is able to send both Tanjiro and Inosuke flying away with his Spatial Manipulation even if they are at sword's reach.
- Inosuke blitzes the Horned Demon who was surprised that a human managed to dodge his attack, and who has managed to injure the Drum Demon even though he spams Spatial Manipulation and invisible sound attacks.
- An injured Tanjiro moves FTE agai.
- Rui shoots webs that slices the Greedy Demon Slayer before he can react.
- Inosuke barely reacts to the transformed Spider Father even though he was previously blitzed
- Nezuko exists the box and moves in front of Tanjiro before Rui's webs can reach him
- Tanjiro dodges Rui with FTE movement
- Nezuko dodges Kanao's attack
- Mom Demon knocks the door and then breaks through it in front of Genya, slashes four of his siblings, jumps to the ceiling, and destroys the light bulb faster than Genya can perceive it. And for the previous objection, yes human can reach superhuman levels by our standards without Breathing techniques, so Genya reacting to the second attack is more than adequately explained by adrenaline.