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.NET 4.5 enhanced the C# and Visual Basic programming languages to make it much simpler to handle tasks asynchronously. The benefit of asynchronous programming applies not just to parallel processing situations, such as when you want to kick off multiple web service calls simultaneously. It also enables your web server to perform more efficiently and reliably under high load conditions. A web server has only a limited number of threads available, and under high load conditions, when all of the threads are in use, incoming requests have to wait until threads are freed up. If your application code doesn't handle tasks like database queries and web service calls asynchronously, many threads are unnecessarily tied up while the server is waiting for an I/O response. This limits the amount of traffic the server can handle under high load conditions. With asynchronous programming, threads that are waiting for a web service or database to return data are freed up to service new requests until the data is received. In a busy web server, hundreds or thousands of requests that would otherwise be waiting for threads to be freed up can then be processed promptly.

As you saw earlier, it's as easy to decrease the number of web servers handling your website as it is to increase them. So, if a server can achieve greater throughput, you don't need as many of them, and you can decrease your costs because you need fewer servers for a given traffic volume than you otherwise would.

Support for the .NET 4.5 asynchronous programming model is included in ASP.NET 4.5 for Web Forms, MVC, and Web API; in Entity Framework 6; and in the Azure Storage API.

Source of Information : Building Cloud Apps With Microsoft Azure