In reality it rarely works this way - there's plenty of systems which are falling apart due to "death of thousand cuts" type issues. You run a profiler and most of the queries are slow and there's no one obvious part to optimize - because developers over the years ignored basic optimizations and there are inefficiencies everywhere.

E.g., for a quick practice run, try optimizing Wordpress without making it a static page via caching - how many queries will you have to optimize and how much of a codebase rewrite will it be to make it significantly more performant?