Cache what you can, never the cart
Product and category pages cache well. Cart, checkout and account pages must not be cached, and a misconfigured full-page cache showing one customer another's basket is the classic disaster.
Rehearse the checkout under load
Test the whole path — add to cart, apply a discount, pay, receive the confirmation email — with realistic concurrency. Payment gateways and mail providers have their own rate limits.
Have a plan for degradation
Decide in advance what you switch off when the site struggles: recommendations, live chat, reviews. Serving a fast checkout with fewer features beats serving everything slowly.
