The other day on my two sites http flood attack was not much more for 3 days has been generated about 1,000,000 page views. Sites themselves are well optimized with nginx cache and wp total cache. The problems were more with the top 10 plug it dynamically finds all the posts constantly pulls php and mysql. Suggest a serious look in the direction of increasing productivity plugin. There are a few ideas.
1. Use the object cache for counters such as at a certain time is down in the settings
2. Use admin-ajax.php to query mysql instead of a separate file