| Keep in mind that there are many layers of complex systems between your JS code and what'll end up happening on your system when it's run. The code defines what the state should look like after its done executing. It expresses your intent. But that code gets transformed several times on the way to being executed and then the hardware can apply mang different possible approaches to executing it when the time comes. Moreso every year, many of those software transformations, as well as the hardware's execution technique, are quite aggressive about revisiting your program's intent with optimizations (of some kind) that make sense within that context. The upshot is that the farther you are from your hardware, the more of these layers there are between your code and its execution, the less influence and insight you have over what actually "physically" happens during execution. When it comes to profiling and optimization of high-level programs like those written in Javascript, this means that it can ve somewhere between hard and impossible to predict how your code changes will actually impact performance. Radical algorithm redesign can often yield salient diffferences that feel largely predictable, but smaller "precision" changes are often going to be a crap shoot. All those layers between you and the hardware were making optimzations already anyway, and your "precision" change may just as easily confound those existing optimizations as well as it might trigger some other. The results are tricky. This is even true in lower-level code, where we're encouraged to do things like inspect compiler output on godbolt or in our compilation output and always confirm our expectations with a profiler (which often proves our guesses wrong). But it's all that much more pronounced in high-level ones. So ultimately, yes, assuming your prevailing algorithms are generally optimal, profiling and optimization is almost always going to feel like a guess-and-test process. But that's okay, because you can test and those tests are usually (not always) telling you if you've made a meaningful difference or not. |