Wordpress sites extremely slow

I have 2 Wordpress sites running WooCommerce and the load time is anywhere from 10s - 40s - they have become extremely slow for some reason. Note, the websites are on VPS have have been running fine for over 12 months. Nothing much has been added to the sites(just 2 products) and no other plugins have been installed. I need someone to diagnose why this is happening. I run ISP config as web server. I have installed caching plugins etc and it has made no difference. Server spec is very high so its not that. I will only pay once the problem is diagnosed and resolved.