A "lightning calculator"

Mental calculators were in great demand in research centers such as CERN before the advent of modern electronic calculators and computers... The world's best mental calculators are invited every two years to compete for the Mental Calculation World Cup... These math prodigies achieve blood flow to parts of the brain responsible for mathematical operations at six to seven times the typical flow... In Frank Herbert's novel Dune, specially trained mental calculators known as Mentats have replaced mechanical computers completely...

More at Wikipedia, including a list of famous human calculators.