A consequence of this concurrent approach is that the peak load on servers can be higher. A single threaded server can only deal with one request at a time. It will therefore typically spawn a sub process to handle each incoming request, to free up the port for another request. Clearly the time taken between first receiving the request and handing it over to another process is critical to the performance of the server. This rate also depends on the load on the server machine from other, unrelated programs.
Servers can vary in their performance degradation under load.
Any additional processing, such as parsing the HTML file to look for server-side includes on the NCSA server, also adds to the time taken to finish servicing the request. A multi-threaded server can handle multiple concurrent requests, so there is no queue of pending requests and clients are less unlikely to time out when sending requests. This should make it easier for new clients and servers to take advantage of multi threading.