I want two types of results: when developing, I want fast turnaround and most of the time don’t care about runtime performance; when deploying, I don’t care much about how long it takes to compile, but I want runtime performance to be optimised.

Even if it's a simple idea, IIRC Fabien Bellard suggested to use very simple compilers such as tcc for prototyping and gcc for final built. A few years ago there was also an intermediate step using clang for error messages (and also portability).

That's not true; for quick turnaround it's important to be able to get results out of the compiler quickly.

In what scenario does executable correctness and performance take a backseat to compiler runtime? Seems backwards to me.

There is a reason that every C/C++ compiler has various optimization options. Developers like the compiler to respond quickly to changes and later they'd like the highest performing code at the cost of compile speed.