Common problem on the WordPress websites that PageSpeed Insights tool from Google reports is “Enable Compression”:

If you are using an Apache server (if you are not sure, you are most probably using Apache, because it’s the most common), there is a quick and easy way to fix this.

You will have to find a .htaccess file in the root of your WordPress installation (the location where you find wp-config.php) and add this code snippet at the end of it (make a backup of the file before making any changes to it):

# ----------------------------------------------------------------------
# | Compression                                                        |
# ----------------------------------------------------------------------

<IfModule mod_deflate.c>

    # Force compression for mangled `Accept-Encoding` request headers
    # https://developer.yahoo.com/blogs/ydn/pushing-beyond-gzipping-25601.html

    <IfModule mod_setenvif.c>
        <IfModule mod_headers.c>
            SetEnvIfNoCase ^(Accept-EncodXng|X-cept-Encoding|X{15}|~{15}|-{15})$ ^((gzip|deflate)\s*,?\s*)+|[X~-]{4,13}$ HAVE_Accept-Encoding
            RequestHeader append Accept-Encoding "gzip,deflate" env=HAVE_Accept-Encoding
        </IfModule>
    </IfModule>

    # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

    # Compress all output labeled with one of the following media types.
    #
    # (!) For Apache versions below version 2.3.7 you don't need to
    # enable `mod_filter` and can remove the `<IfModule mod_filter.c>`
    # and `</IfModule>` lines as `AddOutputFilterByType` is still in
    # the core directives.
    #
    # https://httpd.apache.org/docs/current/mod/mod_filter.html#addoutputfilterbytype

    <IfModule mod_filter.c>
        AddOutputFilterByType DEFLATE "application/atom+xml" \
                                      "application/javascript" \
                                      "application/json" \
                                      "application/ld+json" \
                                      "application/manifest+json" \
                                      "application/rdf+xml" \
                                      "application/rss+xml" \
                                      "application/schema+json" \
                                      "application/vnd.geo+json" \
                                      "application/vnd.ms-fontobject" \
                                      "application/x-font-ttf" \
                                      "application/x-javascript" \
                                      "application/x-web-app-manifest+json" \
                                      "application/xhtml+xml" \
                                      "application/xml" \
                                      "font/collection" \
                                      "font/eot" \
                                      "font/opentype" \
                                      "font/otf" \
                                      "font/ttf" \
                                      "image/bmp" \
                                      "image/svg+xml" \
                                      "image/vnd.microsoft.icon" \
                                      "image/x-icon" \
                                      "text/cache-manifest" \
                                      "text/calendar" \
                                      "text/css" \
                                      "text/html" \
                                      "text/javascript" \
                                      "text/plain" \
                                      "text/markdown" \
                                      "text/vcard" \
                                      "text/vnd.rim.location.xloc" \
                                      "text/vtt" \
                                      "text/x-component" \
                                      "text/x-cross-domain-policy" \
                                      "text/xml"

    </IfModule>

    # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

    # Map the following filename extensions to the specified
    # encoding type in order to make Apache serve the file types
    # with the appropriate `Content-Encoding` response header
    # (do note that this will NOT make Apache compress them!).
    #
    # If these files types would be served without an appropriate
    # `Content-Enable` response header, client applications (e.g.:
    # browsers) wouldn't know that they first need to uncompress
    # the response, and thus, wouldn't be able to understand the
    # content.
    #
    # https://httpd.apache.org/docs/current/mod/mod_mime.html#addencoding

    <IfModule mod_mime.c>
        AddEncoding gzip              svgz
    </IfModule>

</IfModule>

The code snippet is from the HTML5 boilerplate configuration for Apache.