Having done a lot of performance work on Gecko (Firefox), we generally knew where cycles mattered a lot (low level graphics like rasterization, js, dom bindings

Having done a lot of performance work on Gecko (Firefox), we generally knew where cycles mattered a lot (low level graphics like rasterization, js, dom bindings, etc...) and we used every trick there. But for the majority of the millions of LoC of the codebase these details didn't matter like you say. If we had perf issues that showed up outside they were higher level design issues like 1) trying to take a thumbnail of the page at full resolution for a tab thumbnail while loading another tab, not because the thumbnailing code itself was slow, or 2) running slow O(tabs) JS teardown during shutdown when we could run a O(~1) clean up step instead.