dotdeb packages include mod_pagespeed for nginx in wheezy

add dotdeb repos to /etc/apt/sources.list:

deb http://packages.dotdeb.org wheezy all
deb-src http://packages.dotdeb.org wheezy all

update and install nginx-extras from dotdeb:

apt-get update
apt-get install nginx-extras

create cache dir:

mkdir /var/cache/ngx_pagespeed/
chown www-data:www-data /var/cache/ngx_pagespeed/

edit /etc/nginx/sites-available/default:

server {
  #....
  
  pagespeed on;
  pagespeed RewriteLevel CoreFilters;
  pagespeed FileCachePath "/var/cache/ngx_pagespeed/";
  pagespeed EnableFilters combine_css,combine_javascript,remove_comments,collapse_whitespace;
  
  #....
}

play with the filters:

https://developers.google.com/speed/pagespeed/module/config_filters