What are the best practices for making FlexCards performant on a busy Lightning record page?

A record page with several cards multiplies every inefficiency by the number of cards, so the discipline matters more here than anywhere else.

1. Choose the lightest data source that works. Turbo Extract for single-object reads; a standard Extract only when relationships or formulas are genuinely needed; an Integration Procedure when several sources must be combined — in which case combine them once rather than giving each card its own source.
2. Cache deliberately. Reference and configuration data can be cached for a meaningful period. Transactional data usually cannot. Set the duration from how often the data actually changes, not from a default.
3. Return only the fields you render. Data source definitions tend to accumulate fields that were needed once. Every extra field is payload and, for related objects, potentially extra query work.
4. Be careful with repeatable cards. An unbounded list can render a very large number of child components. Apply a sensible limit and give the user a way to see more.
5. Consolidate cards. Three cards each making their own call to the same object is three round trips. One card with three states, or one Integration Procedure feeding a combined card, is one.