I've seen a few times clients with lots of web traffic struggling, and in some cases they had some advance stuff like load-balancing and memcached, and they didn't even have compressed html!
To me is puzzling that many people don't have it when it takes a couple lines of code and saves typically a good deal of of network traffic, Am I missing a downside?.
A lot of the distros on a fresh install have mod_deflate installed and enabled, but if you have a slightly older distro, it may not be. For debian/ubuntu user just probably already installed, just enable it if its not with
a2enmod deflate /etc/init.d/apache2 restart
Centos/RHEL based ones just add the module line into httpd.conf
LoadModule deflate_module modules/mod_deflate.so
If you have an older distro that doesn't include the module then search via package management for it and install it. At this stage though, chances are you want to look into upgrading it for security purposes if nothing else.