A lot of attention gets paid to vo2 max. Essentially that is the measure of how much oxygen your body can use at max efforts. It is a good measure to know but for our purposes it isn't that important. A firefighter friend of mine talked to me about some drills they used to do at the fire house. They would perform drills on their oxygen tanks and whoever finished the drill with the most oxygen still in their tank won.
Think about that. Calm relaxed breathing conserves the tank which in a real fire would allow you to be more productive. Now if we apply that to CrossFit we are trying to get the work done efficiently. So vo2 max is good to know, but not really applicable to the goals of fitness as we look at it.
A well developed aerobic energy system is important to overall health as well as high level conditioning and sports. But the best indicator isn't necessarily vo2 max, instead start to realize at what point you go anaerobic. It is best to have a heart rate tied to this point to help guide your training. Also, there are some ways to have a general guide while training that doesn't require extra equipment. The bottom line here is still that aerobic energy system needs to be trained too, and while CrossFit is often seen as short and intense that needn't always be the case.