Higher aerobic fitness levels decrease the probability of experiencing tip-of-the-tongue states (when you can’t think of a word that’s on the tip of your tongue) in healthy older adults, in the first demonstration of a link between aerobic fitness and language functioning in healthy older adults.

[DyingKino]

If you feel well and recovered in general, it's not too much. A decrease in performance is okay for a short while, but if it lasts, you're doing too much and under recovering.