Alexis Lemaire was declared the faster human calculator when he took the 13th root of a 200 digit number in his head in 70.2 seconds. This was the world record and Alexis was very proud of himself. The first 200 digit number that he tried to 13th root took him 40 minutes to complete in his head. He then put himself through many training exercises to repeatedly cut his time. When asked, Alexis would not give away his secret but he did say that it was not all about math. It was also a lot of memorization. Daily Mail challenged Alexis with a 30 second challenge that he completed, in 8 seconds. 

This really makes you think that maybe its possible to become a human calculator. At first i had thought that it was just a born talent that people had but the way that Alexis puts it, it seems to be more of a self taught skill.

http://www.dailymail.co.uk/sciencetech/article-501232/The-human-calculator-393-trillion-answers--picks-right-70-seconds.html