I remember the Youtube dev story posted here. They optimized Youtube page at some point, deployed and saw much worse average load time for users. As it turned out the whole new batch of users saw load time decrease from unusable tens of seconds or minutes on slow connections to tolerable less than 10-20 seconds or so and they started using the site which dropped average time by a lot :) All these modern developers usually ignore how their websites overloaded with usually unneeded scripts and trackers load on slow networks and PCs.