How does a site like rambler serve dynamic content so fast? ...
Is this purely Nginx’s capability? Where should I be looking into to learn about such capabilities?
This has little to nothing to do with the web server used -- both nginx, IIS and Apache are 'fast enough' an generally do their work within milliseconds. nginx is much faster than Apache, but this merely means the site owner will need fewer servers for the web serving part -- nginx does not transfer data faster to you.
Steve Souders did the original work while at Yahoo, he is now working at Google. His first book "High performance websites" is the best starting point for learning more about making fast websites. The same material that's in his book can be found in this video talk, and these design rules. However, I find that the book is quick to read, and much easier to understand.
You can run the sites through WebPageTest.org's tester -- that will give you a good feeling for the frontend part of these sites, and why they are faster or slower.
I believe that serverfault.com if served from Nginx will be much faster the IIS 7 (assuming db access time to be same in both the case). Is this a fair assumption?
Nope, that is a misunderstanding. :-)