I don't think you can limit the cpu usage of apache from its own settings.
You can try using a separate app like cpulimit (see: how-to):
$ sudo apt-get install cpulimit
$ man cpulimit
You can also try these to optimize the overall performance of your server.
Edit your /etc/apache2/apache2.conf and use these values:
<IfModule mpm_prefork_module> StartServers 2 MinSpareServers 2 MaxSpareServers 3 MaxClients 50 MaxRequestsPerChild 0 </IfModule>
Expand your swap in /etc/dphys-swapfile set:
CONF_SWAPSIZE=512
Then run:
$ sudo dphys-swapfile setup $ sudo dphys-swapfile swapon
Overclock your raspberry (it's safer than it sounds) here:
$ sudo raspi-config
I use it at 950MHz. There is a higher option (1000MHz), but some people on the forum complained about SD corruption with that one.
You can also set the graphic memory to 16 in raspi-config memory_split if you do not use the graphical interface.