Review of More Than 300 Studies Shows Whether Caffeine Can Boost Your Workout (sciencealert.com)

tl;dr: "For the average person, improvements will likely be between about 2 percent and 6 percent."

When it is heavy near zero Celsius windy rain outside, I sometimes drink coffee to force my self to do runout. Coffee makes me want to do something, instead of reading 'Nutrition and metabolism' book on my sofa.