I am looking for suggestions on Apache/Drupal/MySQL (localhost) configuration, to solve my issue. I use Ubuntu servers 8 LTS with classic Apache installation, and perform manual Drupal installation too. After publishing a site for customer review, I notice, that if a site is not visited very often or not visited at all for some time, such site response time increases significantly, up to situations with timeouts. Although, other vhost sites not on drupal on the same server run smoothly. E.g., if a site is rarely used, opening and navigating through out it takes very long times, or even timeouts. Each next page visit opens faster.
I use regular Apache settings and PHP memory up to 512MB, as sometimes Drupal module installations ask for more. Server is with 4GB RAM, and around 10 sites running on it at a time. Drupal for customer review is set without caching and performance settings turned on, thus I consider, that this can be a bottle neck in some cases, but still, if I work on the site, it is quite responsive, even without performance settings turned on.
I understand that there are very many ways of solutions and potential issue roots, but wondering, if there are some constructive ways to track this down to small list of tasks to accomplish better configuration. I've experimented with different Drupal performance tuning solutions found on the net, but it still does not solve issue, if a site is not used frequently. Restarting apache helps for a while.