Scientists prooved: Coffee is doping for chess players!

When not under time pressure.... Doping in Chess

Methylphenidate, modafinil, and caffeine for cognitive enhancement in chess: A double-blind, randomised controlled trial
Excerpt: "Treatment effects on chess performance were not seen if all games (n=3059) were analysed. Only when controlling for game duration as well as when excluding those games lost on time, ...
When not under time pressure, such effects may result in enhanced performance..."