The other important thing the article doesn't mention is that mouth-breathing causes the body to eliminate too much carbon dioxide -- you're literally "over-breathing". Blood CO2 is vital for proper oxygenation.

By dropping CO2 too low, you actually decrease brain and organ performance by binding oxygen too tightly with hemoglobin, starving your organs.

It's actually fairly rare for young people to have too little oxygen in their blood, but really common for them to have too little CO2, from over-breathing. By slowing down breathing, and most importantly, NOSE breathing, you normalize blood CO2 and restore cognitive / organ function.