I have no idea what they do at Facebook, and I don't have to work on the biggest community in the world that often.
The problems I encounter in term of performance for websites/apps are along the lines of pages being super-slow because of the 100 JavaScript frameworks we like to use nowadays for the smallest of projects, my clients uploading 10MB images that get resized via HTML and still load at full-resolution, etc.
As for coding yes—one issue I have is that not using PHP exclusively I get confused by the fact that the opposite of html_entity_decode is htmlentities and not html_entity_encode, and that slows me down for no apparent reason except the fact that 20 years ago one guy added htmlentities and another guy html_entity_decode, and no one thought about aliasing or deprecating one of the two to fix the unintuitive naming.