C is easily the fastest language out there. Its so fast we write other languages in it. Nobody seriously writes web sites in C. Why? Its very easy to screw up in C in ways that are very difficult to detect and it does almost nothing to help you. In short, it eats programmers and generates bugs.
Building a robust, fast application is not about picking the fastest langauge, its about A) maintainability and B) scalability.
Maintainability means it doesn't have a lot of bugs. It means you can quickly add new features and modify existing ones. You want a language that does as much of the work as possible for you and doesn't get in the way. This is why things like Perl, Python, PHP and Ruby are so popular. They were all written with the programmer's convenience in mind over raw performance or tidiness. C was written for raw performance. Java was written for conceptual tidiness.
Scalability means you can go from 10 users to 10,000 users without rewriting the whole thing. That used to mean you wrote the tightest code you can manage, but highly optimized code is usually hard to maintain code. It usually means doing things for the benefit of the computer, not the human and the business. That sacrifices maintainability and you have to tell your boss its going to take 3 months to add a new feature.
Scalability these days is mostly achieved by throwing hardware at it and parallelizing. How many processes and processors and machines can you farm your work out to? If you can achieve that, you can just fire up another cheap cloud computer as you need it. Of course you're going to want to optimize some, but at this scale you get so much more out of implementing a better algorithm than tightening up your code.
For example, I took a sluggish PHP app that was struggling to handle 50 users at a time, switched from Apache with mod_php to lighttpd with load balanced, remote FastCGI processes allowing parallelization with a minimum of code change. Some basic profiling revealed that the PHP framework they used to prototype was dog slow, so it was stripped out. Profiling also suggested a few indexes to make the database queries run faster. End result was a system that could handle thousands of users and more capacity could be added as needed while leaving most of the code implementing the business logic untouched. Took a few weeks, and I don't really know PHP well.
It may be beneficial to reimplement small, sharp pieces in a very fast language, but usually that's already been done for you in the form of an optimized library or tool. For example, your web server. For the complexity and ever-changing needs of business logic the important thing is ease of maintenance and how good your programmers are.
You will find that most of the web is written in PHP, Perl and Python because they are easy to write in, with small, sharp bits written in things like C, Java and exotics like Scala (for example, Twitter). Wikia, for example, is a modified Mediawiki which is written in PHP but it is performant (amongst other reasons) by doing a heroic amount of caching.