The version of IIS which comes with a client OS like Windows 7 has significant restrictions on what it can do, to prevent you from using the cheaper OS as a server.
Starting with IIS 7 (Windows Vista), the behavior changed from previous versions. In previous client versions of IIS, excess requests would throw a 403.9 error message (Access Forbidden: Too many users are connected.). Instead, Windows Vista, 7 and 8 queue excessive requests so that they will be handled gracefully, although there is a maximum number of requests that will be processed simultaneously.
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Windows 7 – IIS 7.5 Concurrent Requests Limit
- Windows 7 Home Starter: 1
- Windows 7 Basic: 1
- Windows 7 Premium: 3
- Windows 7 Ultimate, Professional, Enterprise: 10
To get reasonable performance, you need to use a proper server OS.