Each technology has ways to manage big surges of load. What’s a bigger variable here is whether the person who coded the website did so in a way that takes advantage of the available technology. As well, in my experience .NET is used for advanced web application design. Using .NET lets one code in a more traditional programming interface than one can get with PHP. This allows for not only more advanced applications, but advanced memory usage, both which might explain why a .NET app may take a bit longer to use.