Yahoo homepage is world’s most visited page and at any given point in time, there are quiet a few bucket tests running on the page tracking CTRs/navigation and several other metrics. In fact, an entire team of engineers just work on Yahoo Homepage and keep the doors rolling for advertisers.
Nicholas Zakas, Front End Engineer at Yahoo shares wonderful insights on how the team optimized the page for performance, especially JS loading etc, AJAX Calls, Loss of spriting owing to various customization the site offers etc.
Do spend some time going through the entire presentation. It surely is worth your time (and hopefully will save you from committing mistakes).