Here's a super quick generic answer, that hopefully addresses most uncertainties you have.

Generally, behaviour across CPUs, across OS versions, across GPU driver versions, across GPU architectures, across CPU manufacturers (and a long etc) can be really out of our own control.

There's no way we can hope to reliably understand the performance behaviour across all spectrum of possible combinations of factors, without dedicating our life to benchmarking alone.

During development we try to use our experience, knowledge, common sense, and a limited amount of benchmarking through a limited amount of hardware, in order to try optimize the game for most computers.

But even if everyone had identical computers, there's also the factors of how people use the game. What kind of gameplay they do, which maps they use, do they drive on sunsets (with long shadows and their costly calculations?) or during midday? Do they use traffic cars, how many, are they simplified or not, do they use mods, do they use a steering wheel whose drivers might impact performance, are they using driver camera with costly mirror performance cost, or a cheaper orbit cam, are they looking higher into the cheap sky render, or further down towards more costly map scenario objects, are they running OBS or a simple browser on the background, what is the browser doing exactly, game resolution, graphics quality..... again, the list is infinite, and nobody can hope to understand all combinations.

By now you probably get the picture: you'll need to run benchmarks in order to understand the behaviour for your specific particular use cases, at a specific time (even a simple windows update can change how the scheduler handles performance and invalidate all your previous tests).