Esta é a maneira de ativar o mod_deflate para arquivos html, xml, js, css, php:
# MOD_DEFLATE COMPRESSION
SetOutputFilter DEFLATE
AddOutputFilterByType DEFLATE text/html text/plain text/xml application/xml application/xhtml+xml text/javascript text/css application/x-javascript application/x-httpd-php
DeflateCompressionLevel 9
# This part is optional, it is used for really old browser >
# DEFLATE NOT COMPATIBLE BROWERS
BrowserMatch ^Mozilla/4 gzip-only-text/html
BrowserMatch ^Mozilla/4\.0[678] no-gzip
BrowserMatch \bMSIE !no-gzip !gzip-only-text/html
BrowserMatch \bMSI[E] !no-gzip !gzip-only-text/html
# SET VAR
SetEnvIfNoCase Request_URI \.(?:gif|jpe?g|png)$ no-gzip