Wordly, the only reason I know the answer to this question is because Steve (one of the mods) told me after I asked this same question. His response was:
"tren will make it more difficult to breathe due to the increase of PGF2A which is an inflammatory prostaglandin, that's why you see guys on tren who need more oxygen breathing heavier "
So being that I'm going the tren route next month and I love cardio, I did a little more research.
Theoretically, because tren increases red blood cell production, one would think it would help cardio/endurance. However the prostaglandin in trenbolone causes a bronchospasm, similar to asthma. It also has a tendency to increase LDL while decreasing HDL and you will get high blood pressure as a result. So basically your heart is working twice as hard. The important thing when taking tren is to stay hydrated and eat complex carbs throughout the day because it lowers your glycogen levels.