WP Fastest Cache Premium

Hi,
Ive got a very heavy site (approx 17 pages) and is 5meg. I activated the plugin and although slightly better, not enough to get the site to open quickly. I’m looking for sub 4 seconds. If I buy the premium version will it work.

Have compressed all the images to 200k and below and loaded rocket lazy load too. I purchased another popular version which made no difference, so just trying to make sure before I buy.
Even though Ive remmed the fat on the site, it does have a full page slider on 13 pages (which I didn’t have now) Have deleted any comments and unused plugins and considering deleting Yoast as its quite a large plugin.