Site inaccessible et trop lent !

Salut,

J'ai un server Debian qui depuis un certains temps c'est beaucoup trop lent ce qui m'oblige a le redémarrer.
J'ai a peu près 20000 Visiteurs par jours ( selon google analytics ).

Information sur le serveur :

Système d'exploitation Debian Linux 5.0
Version de Webmin 1.520
Noyau et CPU Linux 2.6.26-2-amd64 sur x86_64
Processor information Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 4 cores
Running processes 224
Moyennes de charge CPU 0.44 (1 min) 0.10 (5 mins) 0.03 (15 mins)
CPU usage 21% user, 13% kernel, 15% IO, 51% idle
Mémoire réelle 7.81 GB total, 150.07 MB utilisé
Ma configuration apache :

# 
# Based upon the NCSA server configuration files originally by Rob McCool. 
# 
# This is the main Apache server configuration file.  It contains the 
# configuration directives that give the server its instructions. 
# See http://httpd.apache.org/docs/2.2/ for detailed information about 
# the directives. 
# 
# Do NOT simply read the instructions in here without understanding 
# what they do.  They're here only as hints or reminders.  If you are unsure 
# consult the online docs. You have been warned.   
# 
# The configuration directives are grouped into three basic sections: 
#  1. Directives that control the operation of the Apache server process as a 
#     whole (the 'global environment'). 
#  2. Directives that define the parameters of the 'main' or 'default' server, 
#     which responds to requests that aren't handled by a virtual host. 
#     These directives also provide default values for the settings 
#     of all virtual hosts. 
#  3. Settings for virtual hosts, which allow Web requests to be sent to 
#     different IP addresses or hostnames and have them handled by the 
#     same Apache server process. 
# 
# Configuration and logfile names: If the filenames you specify for many 
# of the server's control files begin with "/" (or "drive:/" for Win32), the 
# server will use that explicit path.  If the filenames do *not* begin 
# with "/", the value of ServerRoot is prepended -- so "/var/log/apache2/foo.log" 
# with ServerRoot set to "" will be interpreted by the 
# server as "//var/log/apache2/foo.log". 
# 

### Section 1: Global Environment 
# 
# The directives in this section affect the overall operation of Apache, 
# such as the number of concurrent requests it can handle or where it 
# can find its configuration files. 
# 

# 
# ServerRoot: The top of the directory tree under which the server's 
# configuration, error, and log files are kept. 
# 
# NOTE!  If you intend to place this on an NFS (or otherwise network) 
# mounted filesystem then please read the LockFile documentation (available 
# at <URL:http://httpd.apache.org/docs-2.1/mod/mpm_common.html#lockfile>); 
# you will save yourself a lot of trouble. 
# 
# Do NOT add a slash at the end of the directory path. 
# 
ServerRoot "/etc/apache2" 

# 
# The accept serialization lock file MUST BE STORED ON A LOCAL DISK. 
# 
#<IfModule !mpm_winnt.c> 
#<IfModule !mpm_netware.c> 
LockFile /var/lock/apache2/accept.lock 
#</IfModule> 
#</IfModule> 

# 
# PidFile: The file in which the server should record its process 
# identification number when it starts. 
# This needs to be set in /etc/apache2/envvars 
# 
PidFile ${APACHE_PID_FILE} 

# 
# Timeout: The number of seconds before receives and sends time out. 
# 
Timeout 300 

# 
# KeepAlive: Whether or not to allow persistent connections (more than 
# one request per connection). Set to "Off" to deactivate. 
# 
KeepAlive On 

# 
# MaxKeepAliveRequests: The maximum number of requests to allow 
# during a persistent connection. Set to 0 to allow an unlimited amount. 
# We recommend you leave this number high, for maximum performance. 
# 
MaxKeepAliveRequests 100 

# 
# KeepAliveTimeout: Number of seconds to wait for the next request from the 
# same client on the same connection. 
# 
KeepAliveTimeout 15 

## 
## Server-Pool Size Regulation (MPM specific) 
##  

# prefork MPM 
# StartServers: number of server processes to start 
# MinSpareServers: minimum number of server processes which are kept spare 
# MaxSpareServers: maximum number of server processes which are kept spare 
# MaxClients: maximum number of server processes allowed to start 
# MaxRequestsPerChild: maximum number of requests a server process serves 
<IfModule mpm_prefork_module> 
    StartServers          5 
    MinSpareServers       5 
    MaxSpareServers      10 
    MaxClients          150 
    MaxRequestsPerChild   0 
</IfModule> 

# worker MPM 
# StartServers: initial number of server processes to start 
# MaxClients: maximum number of simultaneous client connections 
# MinSpareThreads: minimum number of worker threads which are kept spare 
# MaxSpareThreads: maximum number of worker threads which are kept spare 
# ThreadsPerChild: constant number of worker threads in each server process 
# MaxRequestsPerChild: maximum number of requests a server process serves 
<IfModule mpm_worker_module> 
    StartServers          2 
    MaxClients          150 
    MinSpareThreads      25 
    MaxSpareThreads      75  
    ThreadsPerChild      25 
    MaxRequestsPerChild   0 
</IfModule> 

# These need to be set in /etc/apache2/envvars 
User ${APACHE_RUN_USER} 
Group ${APACHE_RUN_GROUP} 

# 
# AccessFileName: The name of the file to look for in each directory 
# for additional configuration directives.  See also the AllowOverride 
# directive. 
# 

AccessFileName .htaccess 

# 
# The following lines prevent .htaccess and .htpasswd files from being  
# viewed by Web clients.  
# 
<Files ~ "^\.ht"> 
    Order allow,deny 
    Deny from all 
</Files> 

# 
# DefaultType is the default MIME type the server will use for a document 
# if it cannot otherwise determine one, such as from filename extensions. 
# If your server contains mostly text or HTML documents, "text/plain" is 
# a good value.  If most of your content is binary, such as applications 
# or images, you may want to use "application/octet-stream" instead to 
# keep browsers from trying to display binary files as though they are 
# text. 
# 
DefaultType text/plain 


# 
# HostnameLookups: Log the names of clients or just their IP addresses 
# e.g., www.apache.org (on) or 204.62.129.132 (off). 
# The default is off because it'd be overall better for the net if people 
# had to knowingly turn this feature on, since enabling it means that 
# each client request will result in AT LEAST one lookup request to the 
# nameserver. 
# 
HostnameLookups Off 

# ErrorLog: The location of the error log file. 
# If you do not specify an ErrorLog directive within a <VirtualHost> 
# container, error messages relating to that virtual host will be 
# logged here.  If you *do* define an error logfile for a <VirtualHost> 
# container, that host's errors will be logged there and not here. 
# 
ErrorLog /var/log/apache2/error.log 

# 
# LogLevel: Control the number of messages logged to the error_log. 
# Possible values include: debug, info, notice, warn, error, crit, 
# alert, emerg. 
# 
LogLevel warn 

# Include module configuration: 
Include /etc/apache2/mods-enabled/*.load 
Include /etc/apache2/mods-enabled/*.conf 

# Include all the user configurations: 
Include /etc/apache2/httpd.conf 

# Include ports listing 
Include /etc/apache2/ports.conf 

# 
# The following directives define some format nicknames for use with 
# a CustomLog directive (see below). 
# If you are behind a reverse proxy, you might want to change %h into %{X-Forwarded-For}i 
# 
LogFormat "%v:%p %h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" vhost_combined 
LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined 
LogFormat "%h %l %u %t \"%r\" %>s %b" common 
LogFormat "%{Referer}i -> %U" referer 
LogFormat "%{User-agent}i" agent 

# 
# Define an access log for VirtualHosts that don't define their own logfile 
CustomLog /var/log/apache2/other_vhosts_access.log vhost_combined 

# 
# Customizable error responses come in three flavors: 
# 1) plain text 2) local redirects 3) external redirects 
# 
# Some examples: 
#ErrorDocument 500 "The server made a boo boo." 
#ErrorDocument 404 /missing.html 
#ErrorDocument 404 "/cgi-bin/missing_handler.pl" 
#ErrorDocument 402 http://www.example.com/subscription_info.html 
# 

# 
# Putting this all together, we can internationalize error responses. 
# 
# We use Alias to redirect any /error/HTTP_<error>.html.var response to 
# our collection of by-error message multi-language collections.  We use  
# includes to substitute the appropriate text. 
# 
# You can modify the messages' appearance without changing any of the 
# default HTTP_<error>.html.var files by adding the line: 
# 
#   Alias /error/include/ "/your/include/path/" 
# 
# which allows you to create your own set of files by starting with the 
# /usr/share/apache2/error/include/ files and copying them to /your/include/path/,  
# even on a per-VirtualHost basis.  The default include files will display 
# your Apache version number and your ServerAdmin email address regardless 
# of the setting of ServerSignature. 
# 
# The internationalized error documents require mod_alias, mod_include 
# and mod_negotiation.  To activate them, uncomment the following 30 lines. 

#    Alias /error/ "/usr/share/apache2/error/" 
# 
#    <Directory "/usr/share/apache2/error"> 
#        AllowOverride None 
#        Options IncludesNoExec 
#        AddOutputFilter Includes html 
#        AddHandler type-map var 
#        Order allow,deny 
#        Allow from all 
#        LanguagePriority en cs de es fr it nl sv pt-br ro 
#        ForceLanguagePriority Prefer Fallback 
#    </Directory> 
# 
#    ErrorDocument 400 /error/HTTP_BAD_REQUEST.html.var 
#    ErrorDocument 401 /error/HTTP_UNAUTHORIZED.html.var 
#    ErrorDocument 403 /error/HTTP_FORBIDDEN.html.var 
#    ErrorDocument 404 /error/HTTP_NOT_FOUND.html.var 
#    ErrorDocument 405 /error/HTTP_METHOD_NOT_ALLOWED.html.var 
#    ErrorDocument 408 /error/HTTP_REQUEST_TIME_OUT.html.var 
#    ErrorDocument 410 /error/HTTP_GONE.html.var 
#    ErrorDocument 411 /error/HTTP_LENGTH_REQUIRED.html.var 
#    ErrorDocument 412 /error/HTTP_PRECONDITION_FAILED.html.var 
#    ErrorDocument 413 /error/HTTP_REQUEST_ENTITY_TOO_LARGE.html.var 
#    ErrorDocument 414 /error/HTTP_REQUEST_URI_TOO_LARGE.html.var 
#    ErrorDocument 415 /error/HTTP_UNSUPPORTED_MEDIA_TYPE.html.var 
#    ErrorDocument 500 /error/HTTP_INTERNAL_SERVER_ERROR.html.var 
#    ErrorDocument 501 /error/HTTP_NOT_IMPLEMENTED.html.var 
#    ErrorDocument 502 /error/HTTP_BAD_GATEWAY.html.var 
#    ErrorDocument 503 /error/HTTP_SERVICE_UNAVAILABLE.html.var 
#    ErrorDocument 506 /error/HTTP_VARIANT_ALSO_VARIES.html.var 



# Include of directories ignores editors' and dpkg's backup files, 
# see README.Debian for details. 

# Include generic snippets of statements 
Include /etc/apache2/conf.d/ 

# Include the virtual host configurations: 
Include /etc/apache2/sites-enabled/

Qu'est ce que je doit changer dans cette configuartion ?

Merci pour votre aide !