16 second of average page load time

I have been analyzing my site which has a 70% of bounce rate, and so far my conclusion is because the page load time on average is about 16 second (gathering the data from google analytics). I have tried leveraging every single optimization possible via w3 cache, wp-db-admin, smushed the image, minify css, expiry header, and even tested it out using google pagespeed test and was able to get a decent amount of score as well as GTMetrix score.

Can anyone tell me what is wrong with my site? Because when testing it on my connection it works just perfectly fine.