Manual:$wgUseGzip/ja

詳細
When using the file cache, we can store the cached HTML gzipped to save disk space. Pages will then also be served compressed to clients that support it. In older versions of MediaWiki it also controlled if normal (non file-cached) pages were gzipped.

PHP の zlib サポートを有効にする必要があります.

注記
You can test your website compression here:
 * https://www.whatsmyip.org/http-compression-test/