Hello,
When we test your product page we see 1.1-1.5s server response time https://prnt.sc/10diepa
With WP Rocket we usually get 200-300ms that is quite fast. That is why we suggested you use this plugin.
Also, when we check the product page, we get 80 score for mobile that is a perfect result and we don’t know what else can be done to optimize your website https://gyazo.com/5e5b2ca20c5ede38979df82ad3b1dc5e
Kind Regards