Issue with admin-ajax.php, GET cart, and GET checkout in Woocommerce

I'm working on my Woocommerce website, and I'm having trouble with slow 'add to cart' and 'checkout'. For an ecommerce site, this is quite a serious issue as each of these processes takes about 13-15 seconds to load...sometimes even longer. As I checked with GTMetrix and Pingdom (>> Waterfall), it seems that the slow load is mainly due to 'GET cart' for Cart page (about 7.8 seconds according to GTMetrix) and both 'GET checkout' and 'GET cart' for Checkout page (about 11 seconds according to GTMetrix).

Please note on the following:
- I'm using WP Rocket as my cache plugin.
- I'm using Avada as my wordpress theme.
- My site has WPML installed.
- My site is on Inmotion VPS hosting plan.
- My site has Heartbeat Control installed and set to "Allow only on post edit pages" and "60 seconds" for override heartbeat frequency.
- My site's WP Memory Limit is 512MB

Please advise.

John