How to speed up an apache webserver?

We have a PC:

• Intel(R) Core(TM)2 Duo CPU E7300 @ 2.66GHz


• 3 GByte RAM


• HDD: 160 GByte WD1600AAJS-08L7A0

And we have Ubuntu 12.04 on it with apache 2.2.22-1ubuntu1.7. Serving just static HTML files (that are updated in every ~5 minutes..). Users auth via LDAP, and there is some php, but the php part isn't used much.

The daily distinct user base visiting the site is ~30. There are some scripts running on the PC, but not making any bigger load..

The problem: as the user base gets larger, the website seems to get slow when the user is opening up the webpage.

Question: How can we speed up the loading of the webpage?

Apache benchmark said this little server can serve twice as many users as it is using now.. but sometimes the apache get's slow.

workaround: a cronjob for stop; sync; start apache at lunchtime helped a little bit..

We don't have money for upgrading the physical machine, that is not an option :)

UPDATE#1: apache only works via HTTPS! UPDATE#2: We disabled the unneeded apache moduls, any other idea?