I spent two or so months optimizing the crap out of a majestic monolith and went from under 2K RPS when the PM thought, and the team repeatedly reported, that everything had been squeezed as much as it could, then changing the hardware, which got us to less than 3200 RPS, then to 4K RPS after just a few days of tinkering, to 10K RPS with a bit more effort, to 40K RPS a week or so later. "Oh that's, enough, we don't need to go further." I then changed "quite a bit of stuff" which then jumped us to 2M+ RPS, and then a month later, a consistent 40M+ RPS with low latency on a single box and there is still some juice left in the box should we want to go a little harder.
Right now we're not even touching 5% of the capacity of what we can pull from, it was that much of a change, simply by changing how we think about the problems. Moving from the old server to the new server let us jump from around 1800 RPS to a hair over 3000 RPS. Adding more hardware didn't fix our underlying problems. Adding more complexity was just punting the problem down the road. But changing how to think about the problem? _That_ changed the problem. And changed our answer to the problem.