During the recent COVID-19 pandemic, traditional (offline) chess tournaments were prohibited and instead held online. We exploit this as a unique setting to assess the impact of moving offline tasks online on the cognitive performance of individuals. We use the Artificial Intelligence embodied in a powerful chess engine to assess the quality of chess moves and associated errors. Using within-player comparisons, we find a statistically and economically significant decrease in performance when competing online compared to competing offline. Our results suggest that teleworking might have adverse effects on workers performing cognitive tasks.