documentation/docs/gentoo-desktop-setup/post-install/ccache.md

ccache is a program which save compiling cache speeding up recompile's of big software. First install it:

root@host:~# emerge -av dev-util/ccache

And create a configuration file for ccache.

cache_dir = /var/cache/ccache

max_size = 20G
umask = 002
hash_dir = false
compiler_check = %compiler% -dumpversion

compression = true
compression_level = 1

Configure portage to allow ccache for specified packages:

FEATURES="ccache"
CCACHE_DIR="/var/cache/ccache"

Then for every package with which you want to use ccache add them:

sys-kernel/gentoo-kernel enable-ccache.conf
sys-kernel/firefox enable-ccache.conf
...