Here is an article in the Times on how caffeine can significantly improve one’s performance:
Exercise physiologists have studied caffeine’s effects in nearly every iteration: Does it help sprinters? Marathon runners? Cyclists? Rowers? Swimmers? Athletes whose sports involve stopping and starting like tennis players? The answers are yes and yes and yes and yes.
[...] “There is so much data on this that it’s unbelievable,” [a researcher] said. “It’s just unequivocal that caffeine improves performance. It’s been shown in well-respected labs in multiple places around the world.” [...]
For many years, researchers thought the sole reason people could exercise harder and longer after using caffeine was that the compound helped muscles use fat as a fuel, sparing the glycogen stored in muscles and increasing endurance. But there were several hints that something else was going on. For example, caffeine improved performance even in short intense bursts of exercise when endurance is not an issue.
Now, Dr. Tarnopolsky and others report that caffeine increases the power output of muscles by releasing calcium that is stored in muscle. The effect can enable athletes to keep going longer or to go faster in the same length of time. Caffeine also affects the brain’s sensation of exhaustion, that feeling that it’s time to stop, you can’t go on any more. That may be one way it improves endurance, Dr. Tarnopolsky said.
The performance improvement in controlled laboratory settings can be 20 to 25 percent, Dr. Tarnopolsky said. But in the real world, including all comers, the improvement may average about 5 percent, still significant if you want to get your best time or even win a race.