Caching not working: page speed very slow

Hi,

After fixing a related issue of broken images loaded from an old Jetpack CDN in Fastest Cache, now I see that caching is not helping to speed up my site.

Latest GTmetrix Results
PageSpeed Score – 37%
YSlow Score – 61%
Fully Loaded Time – 13.50s
Total Page Size – 5260KB
Requests – 112

I also saw that in my wp-config.php file, Super Cache plugin (I tried around 2 weeks back) has added the caching line and I cannot find Fastest Cache anywhere, so looks like this plugin is not able to cache my site?

/define('WP_CACHE', true); //Added by WP-Cache Manager
define( 'WPCACHEHOME', '/home1/chennaj6/public_html/wp-content/plugins/wp-super-cache/' );

Please help.