jdMorgan - 1:00 am on May 28, 2011 (gmt 0)
Correction and amplification: Apache on it's own can serve some unbelievable amount of queries. even on a very old/slow server. I second looking into database lookups and any large/complex scripts you may have.
If some content is only "mildly-dynamic," then consider caching static copies for a short period of time and serving those cached copies instead of re-generating each page every time it is requested by a client (and doing the requisite DB lookups repeatedly). For example, most sites won't really suffer if the user gets a page that is only stale by a maximum of one minute, if time is the only factor.