Linux – apache2 slow responding (debian)

See here for a good answer:

Apache2 is not starting my webserver

Assuming you have enough RAM to serve the requests, and you are using the prefork multi-processing module, you can use the 2 directives MaxClients and ServerLimit to set the total number of simultaneous requests that will be served;

To check for prefork run apachectl -V | grep MPM and look for the following output;

#  apachectl -V | grep MPM
Server MPM:     Prefork
 -D APACHE_MPM_DIR="server/mpm/prefork"

The MaxClients directive sets the limit on the number of simultaneous requests that will be served.

Any connection attempts over the MaxClients limit will normally be queued, up to a number based on the ListenBacklog directive. Once a child process is freed at the end of a different request, the connection will then be serviced.

But the ServerLimit also puts a maximum limit on the number of processes that apache will spawn to serve requests.

For the prefork MPM, the ServerLimit directive sets the maximum configured value for MaxClients for the lifetime of the Apache process..

hence I would go for something like;

ServerLimit 1000   
MaxClients 1000     

Alternatively for For the worker MPM, this directive in combination with ThreadLimit sets the maximum configured value for MaxClients for the lifetime of the Apache process. Any attempts to change this directive during a restart will be ignored workers ThreadLimit docs here