Symptoms of poor server response time
If you notice that server CPU utilization appears low, but client requests for static pages take a long time to service, your server may be running out of server threads to handle requests.
This situation results when you have more inbound requests than you have Apache threads to handle those requests. New connections queue in the TCP/IP stack listen queue and wait for acceptance from an available thread. As a thread becomes available, it accepts and handles a connection off of the listen queue. Connections can take a long time to reach the beginning of the listen queue. When this condition occurs, the following message will appear in the error log:
Server reached MaxClients setting, consider raising the MaxClients setting
Server ran out of threads to serve requests. Consider raising the ThreadsPerChild setting