Well, I am writing this guide because enabling mod_deflate on Apache 2.4 has become more complex than enabling a single module like it was on Apache 2.2
Now you have to enable 3 modules in httpd.conf for mod_deflate to work properly :
LoadModule deflate_module modules/mod_deflate.so LoadModule headers_module modules/mod_headers.so LoadModule filter_module modules/mod_filter.so
Also you have to enable compression by this config :
AddOutputFilterByType DEFLATE text/html text/plain text/xml text/css text/javascript application/javascript DeflateCompressionLevel 9