How to enable gzip compression in phpBB3
With gzip compression the data transferred between the server and the client is compressed. This decreases the amount of bandwidth used and consequently decreases the page loading time of your site. The downside is that it also increases CPU usage because before sending or receiving the data it has to be respectively compressed or decompressed, which requires some more CPU resources.
You can turn on gzip compression from the administration control panel of your phpBB3 forum. To do this, after you log in to the admin panel, click on the Server settings link on the left side of the General tab page, under the section labeled Server Configuration.
On the Server settings page the first option is Enable GZip compression. Just mark the Yes button for that option and then click on the Submit button at the bottom of the page to save the change.