i have caching via .htaccess on and i think the setup is correctly; usually we have about 300 clients (woocommerce), yesterday we had about 200, they mostly looking for new products (60-65 uniqe single products dropping every sunday 9:30 pm), some of them add items to the cart, some of them deciding to buy and they generate about 40 checkout on short time (15-30 minutes), but i think the most reason of the overload is massive refreshing website by them at 9:30-9:40, after that massive refreshing everything is back to normal and process of shopping is stable...