[nemonemo]

I'd say the dividing line is how much control you have. If there is short burst of stress for the amount of work you are sure to be capable of, the stress is nothing. The problem is with the duration and the uncertainty.

There is also a genetic factor: I remember there was a gene (or a base pair?) that is called "worrier or warrior", and if one has the warrior one, one performs worse without stress than with stress whereas the worrier one is the opposite.