I couldn’t find a better starting quote that is more to the point than that of Alex Viada, a successful hybrid-training coach and founder of Complete Human Performance, says that such high-altitude devices "simulate altitude in the same way sticking your head in a toilet simulates swimming."
Users will claim they are able to breathe much better after a session with the mask, well if you had a pillow stuffed down your throat for a workout and then removed it, yes you will breathe better too.
Atmospheric pressure, including the partial pressure of oxygen, is reduced at high altitudes. Partial pressure being less means there is less oxygen in a liter of air at high altitude vs low altitude. Less oxygen per breath of air results in less oxygenation of your blood leading to less oxygen being delivered to your muscles.
Prolonged exposure to high-altitude air causes the body to increase myoglobin/hemoglobin content and capillary density, as well as increase oxygen transport to muscles. These adaptations could absolutely result in increased performance. Again though, it takes prolonged exposure, not just 40-60 minutes a day, but rather weeks, many weeks at high altitudes.
The reduced partial pressure of air at an altitude is much different than restricting air intake by using a mask. In fact, not a single person with a shred of physiology knowledge will support elevation masks' ability to increase hemoglobin. Elevation masks don't change the partial pressure of incoming air; all they do is simply reduce the total amount of airflow to the lungs. Again, imagine trying to workout with the aforementioned pillow stuffed down your throat, pretty shitty idea. Performance is not limited by the amount of air you get; it's limited by the amount of oxygen in that air, and how you use it.
If you’re looking to achieve the benefits of high altitudes, research shows that living a sedentary life at these high altitudes is the key. Not training there, but rather just living there. Any training should be done at lower altitudes where training may be maximized. Because the air is thinner at high altitudes, your body responds by creating more red blood cells to boost its oxygen-carrying capability levels, even if you're just sitting or sleeping. This increase in oxygen in your blood can help power you through your aerobic workouts, but training at a high altitude will lead to poorer performance and slower times. So even if these high-altitude masks did what they claim (and they do not) you wouldn’t want to train in one, but rather live with one strapped to your face for 20-22 hours a day, 5 days a week. Picking up your kids from school with a mask, shopping, driving, working, sleeping with the mask would be necessary. Scary thought actually, so please save your money and save wearing masks for role-playing and Halloween.
For a fun read about what high altitude masks may accomplish read here: