Anecdotally, optimizing BCC took user visible load time for e.g. the front page from seven seconds back in the day to two or so these days (and it is interactive faster than that). That page spends no significant amount of time at the DB.

Total time I've spent on front end performance in four years: maybe six hours if that.

This is one of those things whose ROI is so staggeringly high that not doing it is just wrong. It is like source control.