As mentioned in someone else's post, it is PortableApps.com. I left it off as I didn't want to be self promotional in the reference. Mine is far from the largest Drupal site, so other sites like Weather.com and The Onion have done more with large-scale scaling in terms of multiple servers and the like. Personally, for PortableApps.com, I've used Advanced CSS/JS Aggregation [1] and LABjs [2] to help quite a bit in terms of caching along with offloading images and static scripts to a CDN (MaxCDN in our case). Moving the MySQL data from a standard server-based instance to Rackspace's Cloud Database instance and the Drupal install from a dedicated server to a cloud server that can be up/downgraded helped as well. All while keeping costs to about the same as the old single dedicated server.
> Mine is far from the largest Drupal site, so other sites like Weather.com and The Onion have done more with large-scale scaling in terms of multiple servers and the like
Don't forget https://www.whitehouse.gov - it's a Drupal 7 site, and I imagine it gets a fair amount of traffic.
Quite true. They did have one of the largest Drupal sites for a while. I'd wager that the articles that explored how they did it would be a little outdated now as I think they were on Drupal 6 when they switched platforms.
There are a lot of great posts and case studies about weather.com, which is a solid example of drupal in a high performance environment.
Performance has been a specialty for me in my own drupal career. Drupal 8 is a whole new ballgame, though. Cache tags are built in from the ground up, and the page render/caching engine is built to mimic a reverse proxy. Rasmus Lerdorf (creator of the PHP language) benchmarked a pre release of drupal 8 at >2500 page views per second with 20 concurrents.
That's it. And I run Drupal on my personal site as well as you noted. I have a handful of other Drupal sites and a couple WordPress sites I help with, too. I'll probably upgrade my personal site first and my internal test sites running on local server instances to play with.
1: https://www.drupal.org/project/advagg 2: https://www.drupal.org/project/labjs