I have few question
In Which nopCommerce version your site is build on ??
do you have any customization ?? or any third party plugins installed ??
have you check with miniprofiler where it taking time ??
when i am visiting your site i get 502 bad gateway error as well.
did you check that your site is keep restarting ??
It may be cased by the number of products which the store has to load on startup on homepage and category pages. Please start with disabling the plugin which shows the "Featured, Latest, Most View and on sales products". I suppose after that store will be loading faster. If so, it will only remain for you to optimizethis plug-in to work if it actually causes a problem.
To enable the mini profiler You can go into the admin area > Configuration > Advanced Setting, and search for the Setting with name "DisplayMiniProfilerInPublicStore". Enable this setting. This will turn on a Mini Profiler, which will display time it took to generate page with information on which operation is taking what time.
Also i agree with Patryk, i think you first need to try with disable all the plugins which is loading those products on home page for featured, latest, most viewed, sale products etc as those will be query whole product catalog and filter the products and showing to you on home page and you have multiple blocks of same category wise so that could be the reason for your slowness but those block also commingling as lazy loading on scrolling of page so may be that is not the reason as well.
i think mini profiler will give you much idea about that where it taking more times.
if its not already showing on your end then try to Once clear your cache and restart the application after that when you open the site, it should show small popup at top left corner of the browser.