As requested/suggested by Mateusz in the forums TurnKey should consider providing Varnish (a caching tool). There is a Debian package, as well as a modules package, so installation should be straight forward. I don't know much about it really though, so I'm not 100% clear how involved it might be.
Note Mateusz explictly suggested this for Magento, but I think it applies to most, if not all, LAMP based apps.