Contest-style rapid numeric calculation is a special case of "math". Many kids in math clubs focus on both — a significant part of contest math is word problems that have to be interpreted correctly and then calculated correctly, under time pressure — but calculation has little to do with higher math other than being able to validate symbolic results.
Fast calculation is about drilling and learning tricks. People can get pretty accurate and fast at simple arithmetic as long as numbers fit (or they've learned tricks to chunk them) into their working memories. If there's no inherent motivation or necessity to maintain those skills after school, they'll decay.
As the article points out, accountants and engineers are likely to be better at being human calculators. I'd guess accountants first, because engineers do more complex calculations and it'd be much more efficient to rely on computers for most of that. Accounting is focused almost entirely on arithmetic that's possible to be really good at without computers, and less on developing complex formulas where it's simpler to write them out on a computer and then let the computer plug in values and solve. For simpler math, inputting and transcribing numbers from a calculator or computer could end up being the limiting factor.
The other aspect to being good at arithmetic is not being particularly good or fast at doing the exact calculation, but rather at estimating what the results should be, so that if the computer gives them the wrong answer, they know almost immediately that they typed something wrong.
Fast calculation is about drilling and learning tricks. People can get pretty accurate and fast at simple arithmetic as long as numbers fit (or they've learned tricks to chunk them) into their working memories. If there's no inherent motivation or necessity to maintain those skills after school, they'll decay.
As the article points out, accountants and engineers are likely to be better at being human calculators. I'd guess accountants first, because engineers do more complex calculations and it'd be much more efficient to rely on computers for most of that. Accounting is focused almost entirely on arithmetic that's possible to be really good at without computers, and less on developing complex formulas where it's simpler to write them out on a computer and then let the computer plug in values and solve. For simpler math, inputting and transcribing numbers from a calculator or computer could end up being the limiting factor.
The other aspect to being good at arithmetic is not being particularly good or fast at doing the exact calculation, but rather at estimating what the results should be, so that if the computer gives them the wrong answer, they know almost immediately that they typed something wrong.