Add Cache-Control Headers https://technumero.com/increase-page-speed-using-htaccess-wordpress/

.htaccess

#
# Paste the following code into your .Htaccess file 
#


# TN - BEGIN Cache-Control Headers
    <ifModule mod_headers.c>
        <filesMatch "\.(ico|jpe?g|png|gif|swf)$">
            Header set Cache-Control "public"
        </filesMatch>
        <filesMatch "\.(css)$">
            Header set Cache-Control "public"
        </filesMatch>
        <filesMatch "\.(js)$">
            Header set Cache-Control "private"
        </filesMatch>
        <filesMatch "\.(x?html?|php)$">
            Header set Cache-Control "private, must-revalidate"
        </filesMatch>
    </ifModule>
# TN - END Cache-Control Headers