Compressing your content saves bandwidth and improves render time, particular on devices with slow internet connections. Compression allows your web server to provide smaller file sizes that load faster for your visitors. Compression of your HTML and CSS files with gzip typically saves around 50 to 70 % of the file size.

Check if GZIP Compression is active on your website

Adding GZIP Compression via your htaccess (Apache)

# compress text, html, javascript, css, xml:
AddOutputFilterByType DEFLATE text/plain
AddOutputFilterByType DEFLATE text/html
AddOutputFilterByType DEFLATE text/xml
AddOutputFilterByType DEFLATE text/css
AddOutputFilterByType DEFLATE application/xml
AddOutputFilterByType DEFLATE application/xhtml+xml
AddOutputFilterByType DEFLATE application/rss+xml
AddOutputFilterByType DEFLATE application/javascript
AddOutputFilterByType DEFLATE application/x-javascript

# Or, compress certain file types by extension:
<files *.html>
SetOutputFilter DEFLATE
</files>

# compress text, html, javascript, css, xml:

AddOutputFilterByType DEFLATE text/plain

AddOutputFilterByType DEFLATE text/html

AddOutputFilterByType DEFLATE text/xml

AddOutputFilterByType DEFLATE text/css

AddOutputFilterByType DEFLATE application/xml

AddOutputFilterByType DEFLATE application/xhtml+xml

AddOutputFilterByType DEFLATE application/rss+xml

AddOutputFilterByType DEFLATE application/javascript

AddOutputFilterByType DEFLATE application/x-javascript

# Or, compress certain file types by extension:

SetOutputFilter DEFLATE

</files>

Adding GZIP Compression on NGINX

gzip on;
gzip_comp_level 2;
gzip_http_version 1.0;
gzip_proxied any;
gzip_min_length 1100;
gzip_buffers 16 8k;
gzip_types text/plain text/html text/css application/x-javascript text/xml application/xml application/xml+rss text/javascript;

# Disable for IE < 6 because there are some known problems
gzip_disable "MSIE [1-6].(?!.*SV1)";

# Add a vary header for downstream proxies to avoid sending cached gzipped files to IE6
gzip_vary on;

gzip on;

gzip_comp_level 2;

gzip_http_version 1.0;

gzip_proxied any;

gzip_min_length 1100;

gzip_buffers 16 8k;

gzip_types text/plain text/html text/css application/x-javascript text/xml application/xml application/xml+rss text/javascript;

# Disable for IE < 6 because there are some known problems

gzip_disable "MSIE [1-6].(?!.*SV1)";

# Add a vary header for downstream proxies to avoid sending cached gzipped files to IE6

gzip_vary on;

Adding GZIP Compression via a WordPress Plugin

A good candidate is the WP Far Future Expiration Plugin ,which not only activates GZIP compression but adds file expiration for various static file types.
Link

Check speed improvement before and after

Check & Add GZIP Browser Compression

Check if GZIP Compression is active on your website

Adding GZIP Compression via your htaccess (Apache)

Adding GZIP Compression on NGINX

Adding GZIP Compression via a WordPress Plugin

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