Hi, Horace.
It is not exactly plugin to be installed. The standards that influence PageSpeed are well explained in the WebPageTest.Org and GTMetrix sites.
In your case, the .HTAACESS file should resolve. Create a file named ".htaaccess" in the main folder of your site with the example code below.
What will happen?
When the browser loads the site, the htaacess file will indicate that some files can be cached by X time.
<IfModule mod_expires.c>
ExpiresActive on
ExpiresDefault "access plus 1 month"
ExpiresByType text/xml "access plus 0 seconds"
ExpiresByType application/xml "access plus 0 seconds"
ExpiresByType application/json "access plus 0 seconds"
ExpiresByType image/x-icon "access plus 1 week"
ExpiresByType image/gif "access plus 1 month"
ExpiresByType image/png "access plus 1 month"
ExpiresByType image/jpeg "access plus 1 month"
ExpiresByType video/ogg "access plus 1 month"
ExpiresByType audio/ogg "access plus 1 month"
ExpiresByType video/mp4 "access plus 1 month"
ExpiresByType video/webm "access plus 1 month"
ExpiresByType application/x-font-ttf "access plus 1 month"
ExpiresByType font/opentype "access plus 1 month"
ExpiresByType application/x-font-woff "access plus 1 month"
ExpiresByType image/svg+xml "access plus 1 month"
ExpiresByType application/vnd.ms-fontobject "access plus 1 month"
ExpiresByType text/css "access plus 1 year"
ExpiresByType application/javascript "access plus 1 year"
</IfModule>