When it comes to boxing and MMA stand up the experience at the absolute top level is vital. A very important part of this experience is that the fighters are able to pace themselves correctly in a fight that is likely to go for multiple rounds. An elite professional boxer like Floyd has had so many 12 round fights that having the "cardio" to go 12 round is just natural to him not only because of his excellent physical shape and conditioning but also because he knows what his body can and can't do and how he needs to pace himself through the fight to last. When they're building up young boxing talents they're often feeding them tough journeymen that are able to last for several rounds with them so that these boxing talents get the experience of how it's like to fight 4, 6, 8 and more rounds. A great example of a superior boxer who lost because he lacked the experience of fighting a durable fighter in a long 12 round fight is Wladimir Klitschko when the tough journeyman Ross Puritty beat the crap out of him and handed him his first loss. Before he fought Puritty Klitschko was never past the 6th round in his career, as an olympic gold medalist and one of the best pure athletic talents in boxing history he was over confident against Puritty who was able to survive the storm and then prevailed in the later rounds, ultimately winning by TKO in the 11th. Klitschko didn't lose because of "cardio" as such because he always had superb conditioning, he lost because he lacked the experience how to pace himself in a 12 round fight. Now in MMA young talents like Conor aren't built up in this way, Conor simply knocked out everyone left and right on his path to the UFC. He was NEVER in a stand up war that would last past the 2nd round (fight against Holloway was mostly grappling, he can't fight Diaz like that obviously). Nate Diaz on the other hand has 10 fights in the UFC that went to decision. It's this dramatic difference in experience that is Conor's problem, not his cardio. Conor probably doesn't have a cardio problem, it's just that in his career he never faced any tough durable fighters that would push him and last until the decision, giving him this vital experience. Conor went out against Nate like a madman and hit him with everything and now he even thinks that he was actually "winning" because of that and all he needs is more cardio. But this is a wrong way of thinking because if he fights like that again even with better cardio he will still have the same problem except that it will maybe happen in the 3rd or 4th round. It would be better for him if he had a tune up fight with some durable fighter and carry him to the decision to see how his body reacts in a real 5 round pro fight but obviously due to the UFC format this isn't possible. ----- TL;DR Conor never fought a stand up war against a durable fighter that would go past the 2nd round, he lacks the experience in how to pace himself in a real fight that is likely to go the distance. It's unlikely that he has a cardio problem, his conditioning is probably top notch.