Running Apache on Gentoo gives you a lot of flexibility. However, out of the box, there are a lot of modules that are not used in many client configurations.
The modules that Apache is built with can be set by APACHE2_MODULES in /etc/make.conf.
Here is a trimmed down version that removes many unused modules.
APACHE2_MODULES="actions alias auth_basic authn_default auth_digest authn_file authz_default authz_host authz_user autoindex cache cgi cgid deflate dir disk_cache env expires file_cache filter headers include info log_config logio mem_cache mime mime_magic negotiation rewrite setenvif status unique_id userdir vhost_alias"
Gentoo lets you configure the MPM in use. The most common ones are prefork and worker, with prefork being the default. You can change this in make.conf.
APACHE2_MPMS="worker"
You can build Apache, or any program, with different settings than used by make.conf. Apache is a good candidate, since it is a primary application in the LAMP stack.
An optimized version using gcc with highest optimizations:
mkdir -p /etc/portage/env/www-servers
Edit apache file in that directory and add this:
CFLAGS="-O3 -pipe -march=native"
CXXFLAGS="${CFLAGS}"