It's a terrible word choice. It would be better to say "By their very nature, scientists become frustrated when test results are delayed. The prompt return of test results is their highest priority."

This subsequent sentence clarifies the nature of her job by introducing a typical scene in her daily routine. The phrase can be replaced by "This means." However, it's more an outcome or possible scenario because she was the only person responsible for running the tests for other scientists. "Consequently" or "As a result" could also be used for a smoother transition.

It doesn’t have any meaning in this context and the sentence would be better without it. The proper use of the expression ‘by definition’ is that something has properties, functions or behaviour that are characteristic of its title or description. ‘By definition chilli sauce is hot and spicy’ i e. It’s not necessary to say that it is, we already know from its name and ingredients. Sometimes the expression is used with a touch of irony : ‘He’s a businessman : by definition he’s open to negotiation with potential customers.’

It's like it's trying to explain More about her role in that company