Some obvious points got missed, and some got brought up, but not stressed enough.
1: You're vaping menthol. Menthol can be an irritant. If your throat is also bothering you, it's a good bet you're causing a very slight inflammation in the mucous membranes and wet tissues that your vapor is in contact with. Inflammation can both make the process of breathing less pleasant as well as decrease the efficiency.
2: The vapor does not consist only of "water vapor" as is erroneously quoted. There's glycerin in that cloud. Try exhaling your vapor for two days onto the same drinking glass. Then wait an hour to allow any water condensation from your breath to evaporate off. Then take that glass and compare it to another that didn't get your vapor. You are coating your lungs with glycerin while vaping. The upside is the glycerin is much more soluble, as compared to cig tar, making it feasible that your body's natural maintenance systems will have a much easier time keeping on top of that problem. You're also not depositing fine particles from combustion into the tar. Nor are you displacing oxygen in your blood with carbon monoxide from combustion. Those are three of the measurable ways vaping is known to not cause the -specific types- of harm that smoking does. Thad doesn't make it harmless.
3: Vaping has a dehydrating effect. Unless you're pretty seriously on top of your hydration level, it's pretty easy to live and operate at a generally lessened hydration state, but not necessarily dire degree of dehydrated. If you are less hydrated than you were, you will absolutely notice a difference in athletic performance on a lot of fronts, cardiovascular being one of the easier ones to notice.
4: There are a whole host of other variables that could be in play. How's your sleep been lately? How old are you? Have your eating habits changed? Run out of multivitamins last week, and forgot to pick up a replacement bottle? etc, etc, etc.
5: If none of the above seems to you to sufficiently pinpoint your issue, (or even if it does, for that matter) a visit to your physician is ALWAYS a good idea when you notice a palpable difference in your overall wellbeing.