It depends what you're optimizing. Mature c++ codebase that is routinely profiled? Large legacy codebase with few tests? Mixed workloads? All hard. But there's

It depends what you're optimizing. Mature c++ codebase that is routinely profiled? Large legacy codebase with few tests? Mixed workloads? All hard. But there's also a lot of easy cases out there - even good engineers might code like monkeys if they're under time pressure and trying to grow the business. I've done a lot of performance work, and while of course no one individual could achieve a 1% savings per month on the entirety of FAANG compute loads, there is lots of code out there that nobody thought would run at scale or be on the critical path of something important, but now it is and it has gotten expensive.