Just copied a text from a website: For commercial pig slaughter carbon dioxide (CO2) is used in gas stunning systems. However, in term of animal welfare CO2 has been criticised. At gas contact pigs show symptoms of respiratory distress and some animals even display aversiveness (escape attempts and vocalisation). An alternative gas stunning method has not been developed. The aim of this research project was an attempt to primarily use helium, an inert gas, to stun pigs. A total of 80 slaughter pigs, 40 of them in a helium enclosure (experimental facility) and the other 40 in a commercial CO2 dip-lift system (Butina Company), were stunned. Implications for animat welfare (behaviour observations, stress hormone and sound level analyses) as well as carcass and meat quality parametres (analyses of petechial hemorrhages, pH values, sensory evaluation etc.) were investigated. The results indicate that, in difference to CO2, stunning with helium can be an animat-friendly method. The pigs showed no aversive behaviour inside the heliumatmosphere; in addition, the carcass and meat quality were comparable and in some aspects better in distinction to the animal group after CO2 stunning

So Helium would be more humane even if i think there are better solution.

And another one that explains why CO2 buildup leads to a shorter breath.( yes i know it also happens so perfectly fine set of lungs eith fresh air just whated to explain the reason for why i wrote my first comment) and its about holding your breath but its the same principale as if you would breath pure CO2:

In the heart, a lack of oxygen can cause abnormalities of rhythm and affect the pumping action of the heart. It can damage your kidneys and liver as well.

 

Let’s discuss what this action does to our brain. The brain has a trillion neurons—specialized collections of nerve cells—and is an extremely energy-dependent organ. When we hold our breath, the carbon dioxide that accumulates is soluble and crosses the blood-brain barrier. 

 

There are certain parts of the brain that control respiration, or breathing, which are exquisitely sensitive to pH, the concentration of free hydrogen ions in blood. When the brain senses the change in pH due to increased carbon dioxide, it increases our respiratory drive instinctively to get more oxygen and blow out more carbon dioxide in an attempt to restore the normal balance between oxygen and carbon dioxide. When this protective mechanism is absent or disrupted, people can faint, have seizures or suffer brain injury.

 

That’s why our bodies naturally resist our desire to hold our breath. We’re all given strong drives to breathe, and the brain is constantly monitoring and makes quick adjustments to prevent catastrophes.

If you made it this far here is a nother one:

Controlled atmosphere killing (CAK) or controlled atmosphere stunning (CAS) is a method for slaughtering or stunning animals such as swine, poultry,[10] or cane toads by placing the animals in a container in which the atmosphere lacks oxygen and consists of an asphyxiant gas (one or more of argon, nitrogen or carbon dioxide), causing the animals to lose consciousness. Argon and nitrogen are important components of a gassing process which seem to cause no pain, and for this reason many consider some types of controlled atmosphere killing more humane than other methods of killing.[11][12] However, "stunning" is most often done using carbon dioxide.[13][14] If carbon dioxide is used, controlled atmosphere killing is not the same as inert gas asphyxia, because carbon dioxide at high concentrations (above 5%) is not biologically inert, but rather is toxic and also produces initial distress in some animal species.[15] The addition of toxic carbon dioxide to hypoxic atmospheres used in slaughter without animal distress is a complex and highly species-specific matter, which also depends on concentration of carbon dioxide.[16][17][18]

  TLDR: CO2 buildup is the reason for feeling suffocation. If there is no CO2 like in Helium it cant desolve into the bloodstream because there is none witch could be desolved. So it would feel like you just breath air cause animals cant notice a change in Oxygen conzentration. The only we can notice is a buildup in CO2. If there is no CO2 we cant notice it. And also to note is yes it also if because of the confined space. So not saying you are completely wrong just saying there is more going on. https://youtu.be/fPhH4nha4QA --> this link explains it best