What features of a website should be written in C++?

That is a very important question, but not a very known subject in the sphere of web development.
Websites with very high performance requirements can take advantage of C/C++ performance by building plugins in C, or sometimes their own interpreter or compiler for a certain web language.
This includes creating your own language if you need.
For example, Facebook built HiHop (http://en.wikipedia.org/wiki/HipHop_for_PHP), a compiler that first converted PHP into C++, then compile the C++ code.
This is reported to have decreased server load by 50%.
Many Companies customize their version of PHP run-time interpreter and the Apache server, to run their code faster.

ActiveX Controls and Browser Plugins Written in C++ like Flash Player and Windows Media Player also you can write isapi extensions for IIS